Pads (amongst friends) are one of the most sought-after gemstone varieties. They have a long tradition and come with an outstanding mystical image. Described as a color merger of the lotus flower and a tropical sunset, they are a famous topic of discourse amongst gem color specialists.
However, anyone who knows how many colors the lotus flower shows (not to talk of a sunset), can imagine the confusion regarding the definition and usage of the term "Padparadscha".
Pads on photo: Delicate, but not difficult
The color effect of a true natural padparadscha is easy to capture.
But, given their extreme value, natural Padparadschas are probably the most faked variety on the Internet. The combination of orange and pink is more a challenge to the photographer's honesty than to his skills.
One will find anything from pinkish lavender to dull brown offered as "natural Padparadschas". Some sellers do not even go through the hassle of "photoshopping" their stones, but simply sell all off-colors with an idea of pink or orange as Padparadscha:
NO PAD: Orange Yellow (may-be)
NO PAD: Orange Red (may-be)
To qualify as true Padparadscha, a natural sapphire must show orange and pink at the same time.
Here we may distinguish two forms of Padparadscha:
Pink and orange merge throughout the stone. The eye is puzzled with the melting color equilibrium. Some might perceive more pink, while others see more orange. Fascinating especially when a natural gems shows more pinkish orange in tungsten light, while being crispy orange pink at day.
Pink and orange are separated. An orange sapphire with pink areas also qualifies as true Padaparadscha (and the other way around). Though Pads with thoroughly merging orange and pink are even higher priced than those with "simple" color zones, the latter can also make very beautiful gems. Color zoning of this type is also an indication that you are looking at a natural unheated Padparadscha. Nevertheless you should insist on a certificate.
Additionally Padparadscha connoisseurs distinguish between "pinkish orange" (orange is the more dominant color) and "orange pink" (pink being more dominant).
Thus, we may define four types of natural Padparadschas as shown below:
1. Orange Pink (merging orange into pink)
2. Orange with pink (pink zones in an orange stone)
3. Pinkish Orange (merging pink into orange)
4. Pink with orange (orange zones in a pink stone)
1. Merging orange in pink
2. Orange with pink zones
3. Merging pink in orange
4. Pink with orange zones
In Asia the "Orange Pink" (1.) is the most valued variety of the four. In Europe it seems "Pinkish Orange" is favoured.
Because of their value Padparadschas have always been the object of heavy treatments. Not only heating but complete "re-coloring" is common. Treated padparadschas are however of little lasting value. Diffusion treated Padparadschas for example are about as a rare and valuable as traffic jams in Colombo.
Any good looking natural Padparadscha that does not cost significant money is a fake (no exceptions).
When buying true Pads on the web, make sure that the stone is graded as "pinkish orange" or "orange pink" by an independent lab.
Though, as mentioned before, natural sapphire comes in a myriad of colors, most people think of sapphire as being blue.
Blue sapphire is in fact the number one in sales of all colored gemstones. Famous are Cornflower, Velvet- and Sky-blue and the rare Kashmir blue. The roots of the latter color lie, of course, in the origin Kashmir. However, since the mines there have run dry the term 'Kashmir' is commonly used for a specifically sleepy and hypnotic type of deep blue (like the 3ct natural Burmese sapphire above).
Blues are routinely ultra-high heated to create this darker kashmir-like shades. Sri Lanka exports containers of an ugly whitish stone ("geuda") which turns into blue sapphire under ultra-high heat. An estimated 95% of all Sri Lanka sapphire is treated. Untreated natural blue Ceylons are a rarity.
Natural Blue sapphire on photo: The chameleon
Anybody who has tried to capture unheated blue sapphire on photo will have realized that it is extremely sensitive to the light source. Far from understanding the color perception of the human eye versus that of a camera, we can only note that blue sapphire is a camera-chameleon.
The slightest change in angle, distance to, or the light itself result in tremendous changes of the captured color. The photos below show the same 3.5 carat blue sapphire without any further editing (other than sizing).
Photos above: Blue sapphire can be photographed "into" many hues.
Depending on the light source and angle the camera transmits different hues and shades. Hence, a "sell & run" trader could offer this stone as cornflower blue, purplish blue or even as a light Kashmir blue - resulting in very different prices.
Yet, the "real" color, meaning the one which the eye perceives, could be described as 'deep steel blue', and is best shown in the last photo (#3 in 2nd row).
However, deviation between reality and the photo are not always the result of bad intention or fraud. Especially light colored sky-, marine-, and steel-blue stones are in fact difficult to capture correctly. Therefore any photo should be accompanied by a straightforward grading and a written description taking up any issue that the photo might withhold from the buyer.
The lesson to learn: When buying natural blue sapphire on the web, always carefully read and consider the seller's description and grading. When the "real" blue might be hard to capture, it is worthwhile to pay attention to all other information the seller gives. (Hence, if we say that a stone is lighter colored or more steel than sky blue, we mean it.)
In Sri Lanka we find particularly bright yellow sapphire. Though not strong in color they do excel in luster and crystal. A well cut light yellow natural sapphire is a delightful lively gemstone.
Very often, these light yellow stones mark the border to white (colorless) sapphire. White sapphires with only an idea of yellow (or any other color) are here called "Tinted White". Such a natural yellow can be just as fascinating as a fancy diamond but will cost only a fraction.
Intensely colored yellow sapphires of bigger size usually come with a tint of orange or green. Thus they can range from a rich canary yellow to an intense greenish lemon hue. However, flashy colors are extremely rare in natural and untreated yellow sapphire.
In the past years light yellow has been the favorite sapphire color amongst collectors, and their prices have soared. Nevertheless they are still affordable when compared to blue or pink.
Natural Yellow Sapphire on Photo: Pardon those inclusions
More than other lightly colored stones, yellow sapphires do not stomach inclusions very well. Even an only "lightly included" yellow may seem to be somehow dirty on a photo, though, in fact, the eye will not perceive any inclusions at all. In deeper colors even the lens would not reveal those fine inclusions, but in a yellow they seem to spoil the photo.
Photos above: Three untreated yellow sapphires suffering heavily in front of the camera, though, in fact they only have minor, hardly visible inclusions.
A natural yellow sapphire free of inclusions on the other hand is a grateful gem to work with. They will shine and sparkle especially when exposed to a little bit of sunlight.
Frequently however their luster is so strong that the camera captures them just as a blurred light source. In this case the photographer has the choice to either show the stone as a somewhat fuzzy shining star, or he has to sacrifice the luster and show the stone from the side only. We usually prefer the shining stars:
Photos above: Three natural yellow sapphires shining beyond the camera's capabilities.
Frequently we also experience that the greenish character of a stone becomes strongly exaggerated by the camera. Some stones in fact turn so green that they are hardly recognizable as yellows anymore. This presents one of the rare cases in which it might be legitimate to manually decrease the amount of green captured by the lens.
Photos above: Natural lemon yellow sapphire that showed mostly green through the camera. Only after reducing the green digitally did the real color prevail.
Again, the primary goal of any selling photo is to present the stone as close to reality as possible, yet the additional description ought to mention any potential deviation between photo and eye perception.
Some cultures and languages use the term "purple" differently. As most North Americans and Europeans, we take purple as a color on its own. Violet, also a mixture of blue and red, lies closer to the blue and is therefore counted into the blue gemstones.
Unheated purple sapphires are far undervalued given the strong color sensation they offer. A good natural purple is as thrilling to the eye as a good blue or violet. Nevertheless they have not been getting much attention until the quest for untreated ruby made many people consider other colors than the classical pink and red.
Yet, besides the terrific but rare electric purple, collectors seem to prefer stones with an undertone rather than fully saturated purple ones. This looks like an exception to the rule "the higher the saturation the higher the price".
A reason for this exception may be found in the way pure purple defies the camera.
Purple sapphire on photo: Poor in Solitude
When looking at images of natural purple sapphire on the web one quickly realizes that they can not compete with the dazzling presentation of good blue, ruby or pink.
Whenever you see a breathtaking image of a purple sapphire it is mostly the secondary hue that gives it the "bang". Pinkish purple, reddish purple or violet/bluish purple are great models, but purple on its own does not perform well in front of the camera.
The following sapphires are fine gems for the eye, yet the image does not transfer them equally well. They are examples of rather "pure" purple in different tones:
Photos above: Fully saturated purple shows somewhat dull or lifeless in front of the camera. Add a tint of secondary color, and that changes dramatically.
The following three stones, all with strong color cross-over, do very well in front of the camera. They immediately catch the eyes attention and leave no doubt about their attractiveness.
Photos above: Purple as/with secondary hue is very photogenic. Of course, these stones are in deed fabulously intense colored but they also show it straight away.
Thus, when buying purple sapphires on the web: Give them some credit!
You might well be surprised how reasonably you have obtained a fully colored untreated sapphire.
Though prices shall not be a function of the photogenic capabilities of a variety, they do influence the market situation. Hence, purples are sold relatively more expensive in the traditional channels than they are sold on the web. This is true for all gems, but especially for the camera-shy purple.
Pink, being a lightly colored form of red sapphire or ruby, has become popular in recent years.
Its colors range from a light lavender rose to the so-called "hot pink", which resembles a vivid bubble-gum hue.
Aside from padparadscha, which is partly orange, natural pink sapphires have become the most expensive variety within the fancies.
Prices of pinks vary greatly with size and color intensity. However, untreated hot pinks of several carats have buyers lined up at the mines. In the wake of this popularity prices of pink spinel have increased as well.
The fact that heart shapes are much more frequent in pink sapphires than in any other color points to the emotional occasions they like to be used for. Especially Japanese buyers love big pink hearts.
Hot pink was once a unique offer from Sri Lanka, but we do not see that continued. Madagascar has taken a dominant position for pinks, but most stones are heat treated and are not clean.
(Remark: Though Madagascan stones are sometimes heated at lower temperatures (600°C), we feel that "treatment is treatment". To distinguish between low and high temperature heating does not help at all, but further complicates the situation for the buyer and increase the confusion in the market. Either a stone has been artificially pampered with, or not. If there was no change, then why was it heated in the first place? "A little pregnant does not count.")
If you want a truly natural pink you will have to search longer and, no doubt, pay more.
Pink on photo: Well, that depends.
Ranging from the most tender-baby-complexion to alarm-button-shocker-hue, natural pink does not allow a simple evaluation.
Light pinks are notoriously difficult to capture. Like yellow they suffer from exaggerated display of inclusions, re-pay good luster with fuzzy images and pretend to have windows where the eye sees none. In fact light pinks are known to have made photographers quit their jobs (or being fired).
Below are three adorable pink sapphires that will stop your breath in person, but they are hard to capture digitally in an attractive manner.
Far from cheap untreated light pinks don't forgive shallow areas, over represent inclusions and either swallow their luster, or turn out fuzzy.
Photos above: Light pinks are mercilessly self-critical.
Strongly colored pinks on the other hand are more than robust. The following shots were immediate photographic "bull-eyes", and do neither exaggerate the stones beauty nor understate their weaknesses. Further around the color wheel this is taken to the extreme when even opaque rubies still make relatively good photos.
Photos above: Hot pinks jump straight into the camera without problems.
As a rule, the more color in a pink the more critical you should be about any flaw you can see on the image. Be wary with hot pinks that look too included or windowed, they probably are. Unless the price reflects the visible flaw and the seller names it for what is it, you might have a bad awakening.
On the other side you can make a good catch if you find a fine, but lightly colored, pink that is undervalued due to its bad photo manners.
Truly colorless sapphires are called "white", and are said to be found exclusively in Sri Lanka.
Fine untreated white sapphires have become rare since they can be turned blue, orange or yellow with high heat, irradiation and various other treatments.
White sapphire rivals diamond in some ways. Thus they were often used as a substitute. However, many people have become aware that they do have their own charm, and since then they are valued far above mere cheap wanna-be diamonds.
Most white sapphires like the pinks, are heated, even if on lower temperatures. Some like to conceal this as "only blow-heat". We don't.
Many whites do have light hues - pink, purple or a tint of blue. The border between a pale blue and a white sapphire with a blue tint is not clearly set. From the point of untreated stones, we define the border in favor of color.
A white sapphire that shows some, say, blue but may not be called a blue sapphire, is here referred to as a "tinted white". Such a tint may be imagined as the lightest of all tones. Clear water in a glass bottle for example or ice leaves an impression of being bluish, or white marble might shine yellowish. However, one wouldn't call this blue or yellow straight away.
The tints in white are in fact often so fine that professional graders can not agree on them. Some labs define such a stone as "faint blue" some tend to call it "colorless". At the end of the day it comes down to your personal perception and taste. In any case we will explicitly mention the faintest idea of color in our comment.
All whites do exhibit color when in colored light of course, but that does not count as a tint.
White sapphire on photo: Capturing the stars.
Here are some examples of sapphires that show various tints, but may still be counted as whites:
Photos above: Three unheated white sapphires with just an idea of color. The pinkish center stone is the same as above. One photo was taken with some sunlight, the other one in very cold dim day light.
When choosing a tinted white sapphire on the web, make sure that the stone does not only show colors resulting from an external light effect. Ask the seller and see for the color definition of the lab certificate if you are not sure.
If there is a tint, and you like it, you might have the chance for a bargain in your color of choice.
At any rate, white sapphires are thankful photogenic models. They sparkle and shine with all might. Surprisingly they are not as sensitive to inclusions as one would expect from the experience with yellow or light pink.
The only difficulty one encounters with whites is to rightly capture their luster. Some well cut whites are so good in throwing back light (which is somehow the life-purpose of any gem) that they can't be photographed from the front. Those stones you will find to be shot from a side angle.
Though not satisfying this is often the only way of capturing the stone without simply having a fuzzy light in, say, oval shape on the picture.
Here are some whites that had to be photographed from the side because their luster was too strong to be captured.
Photos above: Three whites with luster too strong for the camera. Hence they had to be shot from the side.
That of course does not mean every white with a frontal photograph are dull.
The following stones sure do have extra fine luster but they probably dispense the light in a way that does not blind the camera, and/or have been shot in slightly dimmed day light, and thus are less aggressive.
Photos above: Whites allowing frontal shots in dim light.
As so often in gem photography, one can not show all qualities (or flaws) in one shot. If the value of the stone (and thus the invested time) does justify multiple views, a row of photos is probably the best way to overcome this issue.
However, there is an obvious connection between an in-depth multi-angle multi-light photo-analysis and the price of a gem. Thus we do not generally photograph every stone from every angle in all light conditions, but deliver additional photos on request.
At any rate, a seller should be willing to provide you with a written statement or an additional photo if you have doubts about certain feature like color or are worried about a flaw.
A last advice regarding untreated white sapphire: Buy them before prices go up.
Commonly stars are known in ruby and sapphire but they also appear in garnet, spinel and other less known varieties. There are differences between those stars but we will here pretend that nature made all stars equal.
Gemstone phenomena, especially asterism, are a tough call to judge on photo. However, no normal jeweler will be able to show you a fine natural star sapphire, let alone a selection to choose from. Good stars are rare even beyond the normal gemstone rarity. Unless you live in a metropolis or travel to Tucson or Basel, the internet is the only place to compare and buy such a gem.
Looking at images on the web, stars seem to be quite an ugly bunch. Rarely do they show nice colors, often they are zoned, patchy, heavily included, silky, egg-shaped and at times the asterism is hardly visible at all.
And of course you will find many "perfect", "fully colored", giant star sapphires or rubies for a few dollars. These are synthetic or surface diffused or lead-glass filled gems which are mostly worth just as much as they cost.
There is nothing wrong with twenty carat Linde star for fifty dollar, but be wary of those sellers trying to offer them as real.
Here are some typical examples of undisclosed fake stars:
So, they are either ugly or faked?
No, don't be discouraged. Real, natural stars are mind-shaking and heart-breaking.
Many star skeptics have become sworn star fans after their first encounter with fine quality:
Ceylon Star Sapphire
Mogok Star Ruby
12 Ray Burmese Star
Here is what to look for when selecting a star sapphire or ruby online.
The value of any star gem depends strongly on the quality of its asterism, which is defined by (no order):
Symmetry & linearity
Completeness (6 rays mostly)
Travel (smoothness of movement)
Lucidity & Depth
The relative importance of these criteria are questions of personal taste, culture and fashion. Most collectors would perhaps trade in some off-centeredness for good movement, or overlook a meandering leg while frowning at a missing one.
We feel lucidity, travel, position and completeness may be most important and price relevant.
Only then, with decreasing relevance, come:
Finish (top and bottom)
Asterism and color together easily make up 80% of the value of a star (sapphire, ruby or any other variety).
With ten dimensions (as compared to the old 4 Cs) stars are a quite demanding topic. But they are rewarding, too.
Let us tackle each issue separately from the web's point of view:
1. Sharpness is easily shown on the web. Here are three stars (spinel and sapphire) with decreasing sharpness:
Three remarks, though:
There is an ideal distance between spotlight and the gem bringing out the sharpest star. You may assume that the photo captures this point. (unless stated otherwise in the grading report)
In general, a camera shows stars not as sharp as the eye perceives them. Hence, when you see a sharp star on photo you can safely conclude that the star is actually very good in person. If you can just make out the star on the photo, it will be fleeting and in need of a strong single spotlight.
Please be advised that one may well find faked (photo-shoped) sharp stars on Ebay or elsewhere. As always, if it is too good to be true... and never buy without 3rd party certificate.
2. Symmetryand linearity too, are easy: What you see is what you get.
Photo: A rather symmetric white star sapphire with some snaky rays.
3. Completeness: No problem either. Count the legs. If the gem on the photo misses one leg it is likely missing for real (unless the grading says otherwise.)
Photo: Star Sapphire with two missing legs
4. Travel, the ability and smoothness of the star traveling over the stone, is not really possible to show on the web (not yet at least). Until online video technology has advanced further, you will have to rely on the seller's description and check later in person.
Ideally the star follows a light source smoothly, not jerking and jumping, while staying intact and sharp. Don't expect perfection though: Every star has a weak area or two. Value reducing would be a loss of completeness or even a sudden disappearance of the star during movement.
6. Position too, is a tricky business on the internet. In order to show the star on a photo one must bring it close to the center. Thus, on a photo, the star will usually be centered. But that does not mean it actually is.
Photo: Three stars looking halfway centered which are in person off centered:
Even a 100% centered star would need camera and spotlight in the same position (which is next to impossible). Hence, position is always a criterion that one must question further even if the photo looks great.
Sometimes you may see the light coming obviously from the side while the star sits centered, meaning that, when the light is moved up, the star will probably shift off-center. However, this is not 100% conclusive. Mostly you will need to rely on the seller's evaluation.
Whenever useful we try to put a number to off-centeredness: "45 degree off-center" for example indicates that when the spotlight is positioned straight above, the center of the star sits half way down the stone. Ten degree would mean the star is just slightly off center and more than 60 degree would send it nearly over the edge.
6. Lucidity & Depth are connected. In one extreme we will have completely opaque material with the star sitting on the surface. Opaqueness is easy to spot on photos. It looks like a solid piece of material with the rays fixed or painted onto the surface.
Photo: Opaque star ruby with the star confined to the surface.
At the other end of the spectrum we may have highly transparent quality with the rays reaching into the body. One can, however, not expect 100% transparency because needles are necessary to break the light and show the star.
If the needles are very fine and yet the star is clearly visible, then ... There is magic: The rays sway like silver curtains inside this most dense material.
Photo: Semi-Translucent star sapphire with rays reaching into the gem
Most stars however are rather opaque characters and transparency is highly priced.
So much to the evaluation of asterism.
The remaining four points are also relevant in normal gems but some additions are helpful.
7. Color: Strong colors are extremely rare in stars. Even the best star will have a silky, silvery sheen to it which clouds its saturation. This is unavoidable. A star needs needles.
Most stars come in grayish, foggy mild colors. Exceptions, like the one below, exist but they are very costly and yet can not rival the intenseness of a 100% transparent gem:
Novices will be disappointed when expecting to find a neon red ruby or a hot pink sapphire with perfect asterism.
One has to appreciate the silkiness as part of the phenomena, and see the color as an add-on. Even grayish white stars have good value if their asterism is of high quality.
The main problem, however, when judging colors on photo springs from the fact that any spotlight, needed to show the star, affects the character of the color. There is no cure to this.
If the spotlight alters the color too much we often add a "no-star" image to show the gem's "2nd" color. This means one image to show the star, and one more to show the color (without a spotlight) under mixed and diffused light conditions:
However, this is not fully satisfying since star quality and color saturation interact. Again you have to read the sellers description carefully and trust his judgment until you can examine the stone yourself.
There is a trade-off between a weak star and a full color: The finer the needles, the better the color potential, but the weaker the star.
Hence, the worst of all possible stars will be valued close to a cabochon of similar quality. Thinking of ruby, we would, at some point, discard the asterism and cut the cabochon into a facetted gem with that sought-after silky ruby sheen.
8. Clarity: Stars are by nature often more included than normal facetted gemstones (beyond their needle structure). This may be connected to their geological origin as semi-transparent material. However, inclusions should not dominate the overall appearance or hinder the rays from traveling.
Photo: Star padparadscha with orange inclusions and yellow color zones adding beautifully to the sunset theme.
Stars also often display stronger color zones. Here too, one has to be tolerant. As long as the star runs unhindered through those color zones you are still on the good side, even if the price of such a gem must be well below an evenly saturated sample.
9. The shape of a star is limited to round or oval cabochon with rare exceptions.
Photo: Pear shape star sapphire (pair)
Generally, asymmetric shapes or other unevenness will reduce prices or should be re-cut straight away. Stars may come flat or dome shaped but this isn't of much importance since there are no windows or blackouts to fear as in facetted gems.
10. Finally the finish of a star: While the dome must be smooth and evenly polished the bottom is often left rough and uneven, frequently even with unattractive edges, holes and other flaws. Don't worry about it.
Photo: As-good-as-it-gets back of a star sapphire: Unpolished, no over-weight, no flaws.
The only thing that counts in a star sapphire or ruby is the top. Of course, in an ideal world, the bottom too can be pretty but don't count on it.
The bottom does not need to be nice but must be as small as possible, not bulging and thus producing extra weight. Enough bottom to accommodate a setting is all weight you want to pay for. A tiny star sitting on a giant rock of corundum will not be priced per carat.
The most important quality of a star sapphire, ruby (or any other variety) is its asterism
The main criterion of asterism may be lucidity, completeness, position and travel
Lucidity and completeness are possible to judge online
Position and travel are not conclusive by photo only, but need the sellers input
Asterism and color together determine the value of a star gem
Strong colors in transparent stones are highly priced but extremely rare
Some fogginess/silkiness is necessary in every star
Colors may vary depending on the type and intensity of the spotlight
Watch out for over-weight bottoms
Treatment of Star Sapphire & Ruby:
Many gem dealers will claim that stars can not be treated. This is nonsense. They can not be ultra-high-heated above 1200 degree because this might melt the needles. Stars are regularly heated below 1200 degree, lead-glass-healed, diffused or filled with bismuth or other chemicals.
In a more thorough approach let us follow the 4C to evaluate a pair:
Carat (size): Many people get overly focused on searching identical carat weight in pairs. This is not necessary and indeed very rare to find. More relevant is the so called "face", the upside dimension of a gem. Two gems might have the same carat weight and yet very different faces and vice versa.
As a rule of thumb you should be searching for gems in the same weight class, say 2-3 carat and then, more importantly, look at the dimensions of the face. Consider width and length. They are most important since they determine the visible face. A pair with too much deviation in the visible part is no good for jewelry.
How much deviation is too much? This is better looked at in relative, rather than in absolute terms. One millimeter difference on a 20mm pear shape might be acceptable but a catastrophe on a 4mm round gem. Barring exceptions we allow pairs to have max. 5% deviation from each other.
The third dimension, depth, is less relevant for your choice assuming that the face is similar and that there are no bad windows. Depth however is the main parameter for weight. Hence, you may find gemstone pairs with equal face dimensions but very different carat weight. Yet, they can be perfect twins for jewelry if the cut is good enough to cover up missing depth, which leads us to the next point:
Cut: Choosing two different cuts or even shapes, say a trillion and a square in a pair is quite exotic; and it is done rarely (but can be charming).
Generally, however, people want near identical cuts. Ideally a pair has been cut to match. It was thus faceted by the same cutter from the same rough material. With "same mine run & same cutter" one has a good chance of getting two gems with a similar character to begin with.
The more important luster is for a gemstone, the more emphasis will rest on identical cuts. A pair of white diamonds for example needs to be machine cut to be 100% equal (which they mostly are in calibrated sizes). In a less lustrous gem, say a midnight blue sapphire or a brown tourmaline, the details of each facet will not be so influential.
As mentioned before, differences in depth can be covered by a clever cutter but this has limits. Too little depth results in windows and there is no way out. If both gems show similar windows that might be OK, but it is very disharmonic to have one fish-eye and one full body sparkler displayed together.
Color: As always in this trade color beats them all. Ruby, tsavorite or sapphires are primarily bought for their magnificent colors and only secondarily for their luster. Some gems like Mali garnet, titanite or demantoid come with fire and color. As pairs they shall be judged by cut and color in equal parts too.
However, most gems are color-first animals.
Tone: Differences in tone are easier to stomach than differences in color composition (mix of 1st, 2nd and 3rd color). This is true especially in darker varieties. (Remember: Tone is the amount of black/white not the saturation). A deep blue sapphire of medium dark 75 will easily fit with a medium dark 80. But be careful with e.g. light yellow or green. There, even small differences in tone will be very visible.
Remark: We would rather suggest using pairs of completely different colors for jewelry than allow different hues in one color. Using two unconnected colors however is quite exotic and we do it only on request. (How about a neon red spinel set with two chrome green tourmalines for example?)
Hue: Where color is king - in pairs it is double. A pair with different color composition will hardly make a nice pair. Add only a bit of purple to a ruby or a tint of violet to a blue sapphire and you will need many meters (not only a face) to separate them from a red-red ruby or sky blue sapphire. A greenish blue and a bluish green sapphire, or a rose red and an orange red ruby, can not be called a pair at all. As soon as the color is visibly different there is no pair.
Remark: Hence, matching pairs are even rarer in natural gems because they lack the uniformity of treatment. Heat or diffusion brings out similar colors in similar rough. The result is a more consistent color mix. Matched unheated ruby or sapphires are therefore a difficult hunt. In fact, this is one reason for the spread of treated gems: Making a necklace with 50 matching sapphires is only possible on a high budget or with high heat. Similarly the industrial production of 5,000 rings with a similar colored blue gem demands controlled artificial coloring.
Clarity: A matching pair should be on the same clarity level. One may accept a "free of inclusions" next to a "very slightly included", meaning both are clean to the eye, but anything that is visible in jewelry should at least be visible in both gems. If a pair comes from the same mine-run, one can actually find beauty in combining identical types of visible inclusions: individual, charming and easy on the budget (think rutilated topaz in a pair).
Width and length determine the visual similarity of a pair not the weight
Better two windows than one
The more important luster is, the more important are identical cuts
Same-mine-same-cutter pairs are best to start with
Differences in tone are sometimes tolerable but differences in saturation are no-no
If inclusions are tolerated they must be visible in both stones