Glossary and Know-How of the Gem-Trade
(contains not only scientific truth but also opinions and anecdotal evidence)
For details on particular gem types see write-up under 'gems sorted by variety'.
Technical terms of jewelry design and mechanics are best looked up in pinterest.com.
A - B - C - D - E - F - G - H - I - J - K - L - M - N - O - P - Q - R - S - T - U - V - W - XYZ
3*NOS: Quality standard introduced here in 2009 - a gem without treatments, without visible inclusions and without window, fulfills the '3*NOS of Excellence'; an abbreviation for 'no treatment, no inclusions, no window' -> three times 'no' or 3*NOS. Note, a 3*NOS does not automatically justify an 'excellent' as overall grade, though more likely, but the gem might still have other problems like a dodgy color or an asymmetric cut which counts as a flaw even if the lapidary did avoid a window.
Appraisal: Independent 3rd party judgment about the financial value of a gemstone or jewelry. To be distinguished from a 'certificate' a.k.a 'lab-report', and also from a 'grading-report'. Competent appraisals are hard to come by, especially if rare gems are involved. Unique, one-only, gemstones do not have a fixed value, like gold or some standardized diamonds, but depend on the specific market-situation and its participants. For many insiders the value of a gem is the price a seller and a buyer agree upon, nothing more, nothing less. Companies like GemVal analyse sales data and offer some statistic guidance but are always backward looking. 'Appraisals' issued by the selling party are mostly worthless or rather they are nothing but the ask-price the seller has already named. They are only useful for insurance purposes. Equally foolish is a 'suggested retail-value' since the actual list-price is the retail-value, online or offline. Unless you buy parcels there is no special wholesale price for single gemstones.
Adamantine: Fire a.k.a. dispersion (rainbow colored sparks) and brilliancy of diamond. Some gem varieties like sphene, titanite, demantoid or zircon have higher dispersion values than diamond, and are technically 'more' adamantine.
Alloy: Mix of metals to change color or hardness, reduce corrosion or increase luster.
Amorphous: Gemstones without crystal structure such as opal, amber or jade.
Art Deco: Fashion style of the early 20th century, colorful, abstract and geometric designs.
Art Nouveau: Late 19th century style based on natural forms, especially flowers, leaves and soft rolling, weaving shapes.
Allochromatic: Gems that are colored by impurities, meaning in a pure form they would be white or colorless, but are often colored by an additional alien agent, iron, chrome, vanadium etc. Such gems are more often treated to change or strengthen the coloring agent. The opposite are idiochromatic gems, colored only by their main 'ingredient' and thus less susceptible to treatments. Allochromatic gems are found in various colors (think tourmaline or sapphire), while idiochromatic gems are always found within one color range (e.g. peridot).
Asterism: Star-shaped light effects in mostly translucent gems, with at least two intercepting rays (single ray is called 'cat's eye') and up to infinite number of rays (which would be a silky opaque). 12 rays are a practical maximum. Famous are stars in sapphire and ruby, but they occur in many more varieties.
Artificial: Man-made copy of a natural matter as in 'synthetic' or an un-natural creation of a matter not occurring in nature for example 'cubic zirconia'.
Adularescence: Shimmer in moonstone and some opals or other translucent gems.
Baguette: Inedible gem-cut in long rectangular shapes.
Bail: Part of pendant through which chain is connected. Can be single-bail (one loop) or multi-bails (commonly two but more are possible)
Band: The part of a ring that does not carry gems.
Bangle: Wrist ring in one solid piece. Can be closed with clasp or open.
Barrel catch: Strongest but slightly fiddly connection between chains.
Base metal: Non-precious metal like steel or brass (as opposed to gold or platinum)
Bead: Smooth round polished gem usually to string up as necklace.
Bezel: Frame type of setting around gem (alternative to prongs). Generally safer for the gem but more sophisticated in setting and less open to light. Valuable gems in daily-use jewelry should be set in bezels because prongs tend to get caught in textile and bend open, loosing the gemstone. The bezel itself can be adorned with smaller gems, textured or set apart from band by using a different type of metal e.g. white and yellow gold. Very pretty!
Belly: Wide shape in the lower part of the gem. Generally pushing up the total weight. Must not be ugly but is expensive when compared to a ideal cut stone without belly but an equally sized 'face'.
Bi- or Multi-color: Gem exhibiting more than one color in the same light and position. Many gems show different colors in different lights and angles but this is not called 'multicolored'.
Black-out: Parts of a gemstone which do not throw back light and thus seem dark. Also called "extinction".
Bleeding: Unpleasant loss of color and saturation in different light setting.
Blemish: Surface flaw such as chip or indent or even a scratch mark. Relevant to gem's value only if eye-visible from the front. Is not considered an inclusion but should be mentioned in grading report.
Body-color: Main-color of the, well, gem-body; for example a cat's eye gem may have a yellow body-color but the ray is white or silver. May depend on the light it is viewed in. Unless stated otherwise, the body color is judged in natural daylight.
Brilliancy: Light reflection in, or rather 'out of' or 'from' a gemstone. One of the most important qualities of a facetted transparent gem determined by cutting style, polish and inclusions but limited by the optical features of a gem variety, e.g. its 'refractive index' and its 'birefringence'. Some cuts, like emerald-cut or baguette, generally result in low brilliancy and are used to show-off 'only' color, while others, Portugese or princess cuts, aim at highest brilliancy. As a rule: the more facets the better the brilliancy. The term is also used for surface qualities of metals, platinum for example is considered to exhibit higher brilliancy than white gold.
Birefringence: Ability of a gem to separate light into its components, i.e. white light into rainbow colors. Found in Demantoid, Sphene, Diamonds, and some rare varieties like sphalerite. Alternative: 'Double Refraction'
Briolette: 3-D cut in shape of a drop without one side considered the 'face'. Mostly used in dangling pairs as earrings or solitaire as pendant.
Brushed: Un-polished surface on a metal in jewelry. Gives a rough, natural look.
Cabochon: Smooth polished gem without facets, usually single sided when to exhibit stars or cats eyes. Also used on rough gems which are too included or opaque to have luster (when faceted) but have a good enough color to be attractive. The lapidary process is called 'cabbing' as opposed to 'faceting'.
Calibrated: Standardized gems in cut and size. Mostly for cheap and smaller gems and such that are used in mass-produced jewelry. Diamonds for halos, for example, are calibrated.
Carat: Weight dimension for gemstones. Equals 0.2 gram or five carat make one gram. For the visual size of a stone you must also look at the dimensions and shape of the visible 'face'. A round and deep two carat might actually look smaller than a princess cut one carat stone because it has a smaller 'face'. Important for jewelry is the 'face' of a stone not its weight. Ideally you want a stone having a big face but no 'window'.
Cat's eye: Single ray effect similar to 'stars' due to needle inclusions breaking light so as to focus it into one 'burning' ray. Usually in cabochon cut gems.
Certificate/Lab-Report: Gemological report (not "grading report" or "appraisal") describing a particular gemstone. Must be issued by an independent 3rd party company. They start with simple variety identification for $25 and go up to comprehensive scientific studies for several thousand dollars. Lab-reports have become a MUST for valuable gems especially those that are suspect to treatments which are noted in a qualified lab-report.
Chevron: Typical feature of princess and scissor cuts displaying one or several V-shaped facets meeting in the cutlet.
Charm: Single dangling, often round, piece of jewelry with or without gems.
Channel set: Row of equally sized gems (mostly 'calibrated') set 'flush' into a band. Used to support a bigger center stone but can also be the main feature of a ring. Rainbow colored channels can be produced by different color sapphires or tourmalines. Very sweet looking, but a hard work for us especially when only untreated gems are to be used.
Choker: Extra tight necklace that sits high up on the neck, also called collar.
Chalcedony: A type of gem but also a translucent crystal with needles in unordered chaotic form.
Chip: Surface flaw, usually caused by sudden temperature changes during transport or pressure during setting. Must be mentioned in grading report. Heated stones are more likely to chip.
Clean: A gemstone without or with very few inclusions. A gemstone is clean if you need a lens to see any inclusions. Some gems, like ruby or emerald, are rarely clean while others such as amethyst or aquamarine are expected to be clean. 'Clean' may be interchangeable with the term 'eye-clean' (no inclusions without lens). One step down is 'moderately included' (inclusions visible to the eye), one step up is 'free of inclusions' (no inclusions visible under a 10fold lens).
Clarity: Measuring the transparancy of a gem, influenced either by more or less inclusions or the type of crystal, e.g. silky or opaque. Also, classification or grouping of gem varieties into 'Type I' gems with usually high clarity (e.g. aquamarine) to 'Type III' gems with typically low clarity or moderate inclusions (e.g. emerald).
Cleavage: Not related to human physiology but describing the tendency of a gem variety to easily break (cleave) along planes or parallel lines. The 'better' or 'higher' the cleavage, the more difficult is a gem for the lapidary: gems with perfect or high cleavage are more likely to break during setting but also later in jewelry. One of the main issues overlooked when only 'hardness on the mohs' scale' is named as measurement of durability.
Color Change/Shift: A gemstone changing color when moved from natural daylight to artificial light (tungsten, candle, etc.). If the two colors are very close each other on the color wheel (e.g. violet and blue or red and purple) it is more accurate to speak of a color-shift. Most gems show some sort of color change in different lights but only an obvious strong change is justifying an extra charge. It must be mentioned in the "Lab report". Some labs demand a jump on the color-wheel (blue to red) to assert a color-change. Most famous and expensive color change is the Alexandrite, but many other gems can be found with color change, from sapphire to tourmaline, garnet and flourite. Color change is an expensive feature but not necessarily easy to understand. It needs time and patience to be fully appreciated.
Cocktail style: Massive stones set in high and bulky rings. Because of the obvious size but also the tendency to be damaged, Cocktail-rings are usually filled with cheaper gems, such as amethyst or quartz or colorless Topaz.
Color-Zoning: Unequally distributed tone or hue in a gem. Sapphires often exhibit tables with stronger colors. If undesired, the cutter chooses the angle or axis so that the best color is parallel to the face (then the color zone is not visible). Color zoning is only graded when visible in the front-view. Valuable color zoning is possible for example in watermelon tourmaline.
Cooked: Derogative term for a heated gemstone. Cooked gems adorn most ready-to-buy jewelry. It is estimated that 95% of all sapphires are heated, and 99% of all rubies. High temperature melts the crystal structure of a gem on atomic-level, basically producing a new, man-made material. Such gems are to be called 'mutants' because they are not found in nature, they are the Frankensteins amongst the gems.
Comfort band: Rounded inside of a band to avoid painful skin irritations in a ring worn on a daily basis.
Concave cut: Gem with facets that are not flat but rounded inwards. Controversial amongst lapidaries with critical view on hiding inclusions, or artificial distracting from a gem's natural beauty. Opposite of 'convex', outward curved. Special cutting equipment is needed for this style.
Crystal: A good crystal is one of the most mysterious qualities of a gemstone. Connoisseurs do use the term freely yet it is hard to make them define it. Probably it better understood by its antonyms: A bad crystal is dull, cloudy, indifferent, non-thrilling, undefined, ugly, muddy, fuzzy, boring etc. A fine crystal is the icing on a quality gem.
Crown: Upper part of a gem, above the girdle. The crown is the visible part of a gem when seen from the front, also 'face'. Many modern settings show also a side-view of a gem thus it is not only the crown that determines quality, but also the symmetry and color of the lower part. Multicolor gems or color zoning offer interesting party-talk when set with open sides.
Cut: Process and result of faceting and polishing a gemstone.
Cushion: Broadly use term for all cutting in square or rectangular shapes.
Culet: Lowest pinpoint of a facetted gem. A cabochon has no culet.
Dead: Rough that will not have luster after the cut, or a very bad faceted gemstone with no luster, mostly due to inclusions or a very bad "crystal" or dark shades (lower than Medium Dark 80).
Depth: Ratio of a gem's face-width to its depth, e.g. a round 10mm gem with 6mm depth has 60% depth. Normal depth in classic cuts sit between 55% and 75%. Beautiful exception can be found especially in deeper gems because they strengthen the color and add light-effects. Low depth is an indicator (though not conclusive) for a window.
Diamond: Most common, widely known, of all gems. Despite outstanding hardness and good fire, colorless diamonds have been artificially overvalued for over a century and are not a rare mineral. Many business experts consider the story of diamonds-are-forever one of the greatest successes in the history of marketing. Trade monopolies have been eroded over the last decades leading to radical devaluation of standard white diamonds. Only untreated vivid colored diamonds are statistically as rare as comparable quality ruby or emerald. White diamonds are not rare gemstones. The colored gemstone trade and the diamond trade used to be separate industries, the former centered in Bangkok, the latter in Antwerp. As so often, the web is disrupting these structures.
Dichroism: Deviation of rays travelling through a double reflective gemstone such as beryl or topaz. Important for ID-ing gems but also a major factor in a gem's optical appearance. Alternative term: 'Double Absorption'
Dispersion: Most important feature for 'fire' in a gem. High dispersion gems like titanite/sphene or demantoid split white light into rainbow colors. Magic! In combination with high brilliancy, dispersion produces 'fire'.
Diffusion: Nasty surface-only coloring (one type of treatment). Diffused gems are valued at a fraction (under 5%) of an untreated gem.
Dome: Upper part of a cabochon, equivalent of the 'girdle' in facetted gems.
Doublet: Two gem-type glued together, one cheap and one expensive part, pretending to be all expensive. Considered cheating if not disclosed.
Double Refraction: Also called 'DR', is the polarization, or splitting, of one (1) light-ray into two (2) rays while it travels through the gem. A high 'DR' is good for fire and brilliancy since it doubles all rays and their reflections.
Energy: Expression used to describe a gems character on an emotional level. One may clearly perceive a gemstone as tranquil, intense or lively. Not scientific but often helpful. Important for astrology and healing.
Engraving: Carving the surface of metal or soft gems into patterns or images. Etching: Like engraving but with lower depth and less precision e.g. brushing.
Eternity Ring: Ring filled with channel set gemstones 360 degree around.
Extinct: Gem which does not throw back light and thus seems dark. Also called "Black-out". Can be the result of terrible cutting or dark shade.
Eye-Clean: No inclusions are visible to the eye. In colored gemstones also called 'lightly included'. One step up is 'lens-clean' or 'free of inclusion' judged with a 10fold magnification. One class below is 'Moderately Included' when inclusions can be seen with the unaided eye but do not yet endanger the gem's durability. Unacceptable in most cases is 'Heavily Included'.
Face: The upper dimensions of a stone which is visible after being set in jewelry. Sometimes also referred to as 'table'. The face is the most important feature to look at when judging a gemstone's size for jewelry. Many flaws are considered non-existent if not visible in the face.
Faceting: Cutting and polishing a rough gemstone with flat (or concave) facets.
Facet-quality: As opposed to cabochon-quality, means a gem is clean enough to be faceted and produce good luster. Of a hundred rough gems found, only a hand-full will be facet-quality.
Fancy: Umbrella term for all that has no name of its own, fancy colors, fancy cuts etc. The opposite may be 'common' or 'traditional' or 'regular'.
Fire: 'Brilliancy' and 'dispersion' together produce 'fire', the former reflects light back out of the gem, the latter splits light into components, especially white light into rainbow colors.
Fish-eye: Term for a gemstone with a big "window". For diamonds the term is also used when a pavilion is cut too shallow, allowing the girdle reflection to show through the crown.
Flush: Setting style in which the gem looks as if dropped into the metal, similar to a bezel in function and security but not necessarily with a frame around the gem.
Fissure & F-Filling: Surface fracture or crevice in a gem. Strictly spoken, not an inclusion (which is not surface-reaching). Emerald and rubies are prime examples for gems common with fissures. Filling such fissures with colored substances, from glue to lead-glass to polymer, is standard treatment practice. If disclosed such 'fissure filling' it reduces the value of a gem to a fraction of its untreated cousin. Undisclosed it is considered cheating. 95% of all rubies in jewelry are thought to be filled. Colorless surface oil in emerald is the only accepted form of treatment at Wild Fish because it is reversible and does not influence color or durability of the emerald. Many other fissure filled gems (such as ruby) dissolve in heat or under cleaning materials, or tend to break under pressure. Having a multi-k-$ ruby reduced to rubble by hot water may count to the biggest disappointments in gemstone collecting.
Fracture & F-Filling: Simply put, a fracture is an especially deep fissure, but as a verb also the physical ability of any matter to break but not 'cleave'. In grading terms, the word 'fracture' may point at durability problems during setting. Internal fractures (not surface reaching) are called inclusions, such as feathers or waterlilies and are NOT a durability problem. Under high pressure and heat, fractures can be melted to increase a gems clarity. The status of such internal fractures are the main indicator for heat treatment as they melt or break during the process.
Filigree: Delicate design of jewelry with thin patterns often with open spaces in-between. Low weight alternative to solid metal with engravings.
Fluorescence: Though scientifically not correct, the term is use to describe a gems (especially a rubies) capabilities to 'glow on its own'. In fact, the stone re-emits ultraviolet light. Related to 'neon' or 'electric'.
Gallery: Part of jewelry in which the gem is held. Can be open or closed for side-view.
Gemologist: Anybody knowing enough about gems. Not a protected profession.
Grading: Process of evaluation by a 'gemologist', in particular the owner of the stone and thus including subjective non-qualitative characterization and overall rankings. Hence 'grading' is to be differentiated from 'certification' or a 'lab-report' in which a 3rd party testifies the stone's quantifiable features or a financial 'appraisal'. Expensive lab reports may include judgements of color, quality of cut or rarity of a gem based on market research.
Gravity, Specific Gravity (SG): Density of a gem in comparison to water. Some gems are heavier (ruby) than others (beryl), thus they produce more weight in less space. Or, said differently, you need more ruby to have a big gem than you need beryl. Corundum (sapphire and ruby) is amongst the heaviest of all gems (heavier than diamond, too). Feather weights are kunzite, opals, quartz and beryls. Heavy weights are all garnets, spinel, zircon, corundum, chrysoberyl. Diamond is somewhere in the middle, surprisingly, attesting that density and hardness are not related. If you are looking for a biiiig gem on a budget, SG or density is worth considering.
Grown: Term used to skip around the fact that a gem is synthetic. A 'grown' sapphire is a man-made copy of natural corundum with little value compared to the real item. Grown sapphires or rubies are sold for cents per gram. Synthetic rough gems are taken to mining areas and then heavily mistreated, cracked, burned and scared to make them look like 'natural' rough. In this dirty state the synthetic may look very similar to the real rough. Especially tricky is the detection of synthetic sapphires after ultra-high-heat treatment because the extreme heat covers the tell-tale signs of synthetic materials, which is then offered as 'heat-only' stones. Modern synthetics are also grown with 'natural'-looking inclusions to further confuse a buyer.
Girdle: Meeting line between crown and pavilion in a gem, or upper and lower part. Diameter or length/width are measured at the girdle of a gemstone.
Hammered: Method of decorating gold (or other metals) by hammer strokes resulting in a rough or antique look. Can be very subtle down to an effect closer to 'brushing', or very obvious with clearly visible regular or irregular deep dents in the surface.
Hardness: Often over-estimated measure of durability in gems. Usually along the mohs' scale from 1 to 10. Brittleness and cleavage are similarly important for the usability of gems in jewelry. Diamond's top hardness has been the main driver of the 'marketing' against less hard gemstones. Yet, protective setting styles can render softer gems equally "for-ever" as diamonds. Pearls, coral or amber are example of very soft gems that have been used in jewelry for millennia. Tourmaline, garnet, spinel, topaz, quartz, sphene... they all are hard enough if set correctly (which we can assure, if we have the honor). EVERY gem, even diamonds, will suffer if set exposed and maltreated daily. The same is true the other way around, every gem can be worn daily if set protected and treated with respect.
Heated: Gemstone treated with heat (from very low to ultra-high) to enhance color or clarity. 'Heat-only' is considered the second-best after 'untreated'. Since most treatments involve heat, the statement that 'no evidence of heat-treatment has been found' does also exclude the present of diffusion or fissure filling. The term 'untreated' is scientifically more significant yet for rubies and sapphires 'heat' or 'no-heat' have remained common wording.
Heat-Only: Gemstone that has only been treated with heat but no other process such as diffusion or filling. 'Heat-only' is the 2nd best class of gems after natural untreated or unheated gems. A gem that is disclosed as 'heated' may well be treated with other value-diminishing processes and is to be questioned.
Hunting: Search for an individual gemstone based on a buyer specification of size, color, cut by a professional trader, either virtual through the trader's sources or on-site in mining countries.
Hue: Main body color, first sight impression, usually without consideration of 2nd colors.
Hypnotic: Term used for the very best colors. Such a hue will imprint itself into your memory and will never be forgotten. A compliment for any gem.
Imitations: Anything pretending to be a gem without being one. Can be a synthetic reproduction or a similar looking material.
Imperial: Expression used on best qualities inside a gem variety: Imperial Topaz, Imperial Jade etc. un-official and subjective term as 'royal blue'
Impregnation: Treatment focused to improve the surface durability of a gem, such as turquoise or jade. In the neutral sense of making a gem more durable this type of treatment is not deceptive but protective. However, the materials used to impregnate the surface often contain color elements to change the body color e.g. green wax on emerald. Undisclosed, such treatment is considered deceptive.
In-the-color: Sales term to describe the quality of a color as being best in that hue. Subjective judgment. Also, since the best color may be changing over time, 'in the color' today may be 'off-color' in 20 years.
Inclusion: Anything that hinders light falling through and out of the stone. Some inclusions are desired (horsetails in Demantoid, needles to produce cat's eyes, light silk in sapphires), neutral (growth-lines) or negative (cracks). Some can be very aesthetic (water lily) others are not (black spots). If you can't see an inclusion with a 10fold lens, it does not exist for grading purposes. Inclusions visible only under a 10fold lens are called eye-clean. If a stone has no inclusions under the microscope (rare) it can sometimes not be distinguished from a synthetic gem. The condition of inclusions are essential to determine treatments, e.g. melted feathers or exploded crystals.
Inspection period: Agreed upon period in which a buyer can decide to return the stone or a parcel after first receiving it. Shipping periods do NOT fall under inspection time.
Irradiation: Hard to detect treatment method to change or strengthen color. Often used in combination with heat treatments. Green diamond and London Blue Topaz are nearly always irradiated. In rare cases, irradiation, like heat, may have been a natural process (pre-mining). Cat's eye gems have also been reported with heavy irradiation. Health concerns have been voiced but are disputed. Sometimes not stable.
Iridescence: Like dispersion, producing a rainbow schiller, but usually near surface. Opals, sunstones and tourmalines are found with iridescent effects. Very rare, ultra-fascinating collector stuff.
Lab: Short for independent scientific institute in gemology. Though mostly for-profit with educational programs and report services they are expected to be impartial in their judgements. Bribery scandals have been reported in diamond grading.
Lab-Created: Synthetic gem, 'grown' rough crystals or bowls of chemically identical copies of natural gems. Mostly detected by their perfection, though processes have been developed to induce natural looking inclusions into the synthetic or they are broken and mistreated to look like rough gems and sold at the mines.
Lapidary: Person or company faceting gemstones.
Lens or loupe: Small, handheld and mostly foldable, glass with 10 (standard for grading) or 20fold (sophisticated) magnifying power. Inseparable from any professional gem trader and collector. Should be neutral in frame color and a triplet. Cost: between $2 and $100, with $20 doing fine.
Lot: Big "parcel" of gemstones. Serious wholesale parcel, perhaps with different gem varieties mixed.
Luster: Term to describe the optical qualities of materials, together with brilliancy and fire though not mutually inclusive, also surface shine in metals or pearls.
Mine-run: Most professional mines are organized in time-phases based on weather seasons or work cycles. For example, a mine team may dig out gem bearing gravel for one week and then wash the gravel in the next week and thus concluding one mine-run. Many mines actually have annual mine-runs, e.g. digging in the dry season or in summer and then washing in the rainy season.
Millegrain: Engraving style on surface of metal. Has a brilliant sparkle to it and can be used instead of diamond-halo.
Mohs' scale: Hardness of gem measured non-linear from one to ten. Over four a material is considered hard enough to be used as a gemstone. However, brittleness and breakage must be considered as well. Even the hardest material can be damaged in its particular weakness, and any soft material may be set in jewelry so as to be protected from outside pressure. The position of jewelry on the body is equally important. Obviously a ring is under most duress, while a pendant worn on the skin will not be damaged unless a serious accident occurs. Once a gemstone is set in a pendant the stone can be considered as safe as the neck it lays on.
Mounting: The yet empty jewelry where the gem will be placed, also called finding.
Natural gem: Gemstone formed in geological processes without the help of humans or simply: non-synthetic. The term 'natural' alone does NOT mean untreated. It is often mis-used to give the impression of non-treatment. A natural gem can be heated, diffused or colored and still is 'natural'. Only the term 'natural untreated' does imply the lack of any artificial improvement. Obviously this scientifically correct term is not very practical, nor marketing-friendly. Most treatments involve heat, therefore the trade often calls natural untreated gems simply unheated which is more or less exact.
Off-center: Cut in which the back does not close in the middle of the gemstone but off the center. The 'cutlet' is then not centric or 'off-center'. This happens when the lapidary, in order to keep weight and depth, follows the natural shape of the rough stone. The alternative to being off-center is usually a "window" and much less weight. The luster of such stones often runs unequal or leans towards one side. Nevertheless, being slightly off-center is not considered a major flaw but only a lesser evil as compared to a "fish eye" in half the size.
Off-color: Colors outside the classics like cornflower blue, maize yellow or rose red. Off-color does not mean ugly or unattractive but simple currently not in fashion in a specific variety. Off-colors can be good investments and are often just as beautiful as classical hues. A color that is most desired in one variety can be off-color in another variety, like yellow brown chrysoberyl cats eye (in) compared to yellow brown sapphire (out). Brown and, to an extend, purple are currently unpopular and cheap, but may have been very valuable in different times for example purple was long considered the most royal of all colors.
Offer: Making an offer is to name and commit to a price you are willing to pay for a gemstone. Taking back an offer without very good reason is deemed impolite in the trade and might result in being shunned next time.
Opaque: No light falls directly through a stone. Some varieties are opaque by definition (pearl) while others may be just very included. Opaque gems may still display color effects like iridescence or schiller. They can also be color-changing. If some light falls through a gem it is called translucent, if it is 100% clear transparent. The word semi- is used to describe borderline cases.
Origin: The place where a gem is mined. An origin can be different from the "country of manufacturing" which, legally and for tax purposes, is defined as the place where a stone is "cut". Premium origins like Ceylon or Burma command higher prices.
Pad: Short-form for padparadscha, a rare light orange pink sapphire.
Pale: Other word for light tone or shade but with a negative notion. A pale stone has less saturation than a light stone, thus potentially more grey than only a lack of color.
Pavilion: Lower part of a gem, under the crown. The girdle separates crown and pavilion.
Pave: Many small stones set to look like an encrusted surface.
Parcel: More than three gemstones sold for a package price. As a rule, if you negotiate a parcel price you cannot later cherry-pick from the parcel but you either have to take the complete parcel or reject it.
Phenomena: Special light effects such as colorchange, cat's eyes, or stars in gems
Pleochroism: Multi-refractive gems will split light into several distinct rays and colors, thus creating several colors in one gem, e.g. blue from one position and purple from another.
Pigeon Blood: Ancient, slightly over-used, term for the best ruby red, a deep violet red sometimes attributed only to Burmese rubies but in fact also found in other locations.
Point: 0.01 carat. Measure used for smallest stones (to make it sound bigger). For example 10 points is 0.10 carat, which is really small but doesn't sound so bad. Mostly used for diamonds.
Polish: Final smoothing of facets in a gem, or surfaces in metals. Badly done, this final step may well ruin a fine cut and leave a good crystal dull. Multi-step process with sophisticated materials depending on the type of gemstone.
Prong: Claw-like metal-pol touching a gem in one place. Several prongs hold gem in jewelry, as opposed to bezel where a frame hold the gems along stretches. Prongs are generally weaker and prone to hook-up and bend-out, thus loosing the gem. Nevertheless, they are preferred technic to maximize light intake of gem.
Precision cut: Modern perfect CAD-based cutting methods that have arisen only in the last two decades. Read more here.
Princess or Scissor Cut: Sophisticated precision cut (square or rectangular) that was originally developed for diamonds but is often asked for in other gems. It is less deep (50%+) than other designs and thus uses a significant more rough material than cuts with belly or rounded pavilions.
Recovery: Percentage of a gem that remains after cutting and polishing the rough. A realistic recovery rate is 20%. Thus one needs one gram of "rough" to make a carat of final gem. The term is also used to describe the loss in re-cuts, e.g. from an emerald cut into a scissor.
Refractive Index: Bending factor, breaking degree, of a ray of light entering a gemstone. Important to ID a gem but also relevant for its brilliancy, cutting options, ideal-cut and much more.
Rutilated: Type of visible needle inclusions. Originally from Rutile, but the term is used for other type of needle inclusions as well, e.g. Ludwigite Needles in Peridot. Very exotic.
Rough: Uncut unpolished gemstone including crystals. Often illegal to export from 3rd world countries without special permission. Crystals are rough gems considered too pretty to be cut.
Rose gold: Fashionable alloy of gold and copper, pink or light rose in color.
Saturation: A 100% red saturation theoretical means there is nothing but red in a gem. In reality there are always secondary hues and grey tones. However, even 80% saturation is very pure and you will probably not recognize any other color or grey.
Semi-precious: Criticized term for less valuable gems. Most people in the trade agree that it has no more meaning but continue to use it to differentiate between high-end collector stones and cheaper gem material. Some previously semi-precious stones like red spinel or chrysoberyl have sure entered the precious realm. However, just because something is called sapphire or ruby does not mean that it is precious. In fact 95% of all that is sold as ruby or sapphire does not even qualify as semi-precious while some amethyst or garnet may be relative cheap but can be high-end collector items and are surely precious.
Semi-mount: Finding ring that already has sidestones but not the centerstone.
Setting: Making a gemstone wearable in jewelry.
Shape: General outer form, like square or oval, of a gemstone.
Shank: The part of the band in a ring without gem (equal to 'band')
Sleepiness: Special quality attributed to high-end rubies and Kashmir blue sapphires due to very fine invisible needles shattering light rays. The novice might easily mistake it for lack-luster but is in fact a compliment.
Sustainability: Usually describing non-destructive and maintainable use of natural resources, as opposed to overexploitation. In the gem trade with basically non-renewable resources the term is used to express concern for environmental and social welfare in the mining countries. Non-sustainable mining would describe the exploitation of gem resources without improving the local life conditions and leaving the place polluted in the process.
Solitaire: A piece of jewelry with one gem only.
Simulant: Gem or material pretending to be something else, like green glass offered as emerald. As opposed to:
Synthetic Gem: Man-made reproduction of a natural gem.
Symmetry: Precise and ordered lay-out of a gem-cut, with exact centered cutlet, equal sides, facet lengths. etc. In perfection only possible for precision cuts.
Step-Cut: Large center facet with smaller facets attached. More in use for color-gems, not for brilliancy.
Star-Gems: See Asterism
Table: The one biggest facet on top of the gem, or its crown. Not equal to face which is captured by the girdle.
Tint: A fleeting idea of color in a white gemstone. In lab reports often called 'near colorless' or 'very light'.
Tone: Degree of lightness or darkness. A lightest tone will be nearly colorless (white/grey) with just a "tint" of color, while a very dark tone will be nearly black. Ideal tones vary from variety to variety, e.g. pads are said to be best in a light tone while blue sapphire can be rather dark. Tone 0 is white, and Tone 100 is black.
Translucent: Half-way between opaque and transparent. Often found in cabochons with star or cat's eyes. Also in color-based gems like emerald or ruby. Light is diffused but not stopped as it travels through the gem.
Treatment: Any alteration of a natural gem to improve its appearance except cutting and polish. The most common treatments are heating, irradiation, fracture fillings and lately Beryllium treatment. The border between treating and cheating are flux. It is considered permissible to sell heated sapphires as long as the stone is disclosed as being treated. Especially stones in jewelry are however rarely labeled correctly but are nearly always heavily 'cooked'. Though not, yet, illegal, Beryllium treatment, filling or irradiation without disclosure can be called cheating because a gem trader sells something practically worthless or potentially dangerous as precious. Often such gems are mixed under high-value untreated gems and increase the overall margin exponentially. Gemstones prone to treatment should never be bought without competent 3rd party testing. Read more here.
Unheated: Gemstone that has not undergone heat-treatment over, say, 100 degrees Celsius. Cutting and polishing does also increase the temperature of a gem, as does intense staring (kidding), but it will not change the gemstone's appearance. Because heat is the main indicator for many other treatments the trade often uses the terms 'unheated' and 'untreated' as equals though this is not 100% correct.
Untreated Natural Gem: Non-synthetic gem that has not undergone any man-made changes other than faceting and polishing. Type of gemstone maintaining its value after you bought it.
White: Colorless. Includes also 'tinted' gems, which are almost white but with a hint of color. Few gems are 100% white.
Window: See-through center of a shallow stone. Windows are disturbing in light colored stone because they create a "fish-eye" effect and do not reflect light in the center. The darker or more intense a color, the less a window is to be worried about. You may snatch a color bargain if you don't bother about a window. Also, there will always be a trade-off between a window and a big face: the bigger a window, the bigger the face of a gem (visible upper part) with the same carat weight to pay for.
Zoned / color zoning: Layers or patches of color or areas of no color in a gemstone. Mostly unwanted, unless in multicolor gemstones where the shape, regularity and clearness of color zones is highly priced e.g. in watermelon tourmaline.