Obviously the sun is the most natural of all light sources, but most stone are worn rather indoors and under artificial light than while sunbathing. Therefore it is important to see a stone both in daylight and in artificial light.
For tungsten light you should choose a strong, but realistic beam. Any stone will shine under the spotlights of an Academy Award ceremony (but you'll also need 2.5 kilo of make-up to look halfway human). For most of us, though, that's not a day-to-day situation.
A torch or a strong ceiling light should be enough to get a good stone to sparkle. The strong halogen spotlights used by some jewelers to sell their stones are stunning but will lead to disappointments once you take the stone home, so try to see the stone in a realistic surroundings.
Unless you actually plan to wear your stones on the beach you don't need to wait for a sunny day. It will be sufficient to examine the gem at noon next to a bright untinted window.
For cat's eyes and stars you will have to make sure that only one light source rests on the stone or the effect will blur.
If you choose a stone to be set in jewelry you will have to consider the setting and the light effects it will have. Will the back be closed or will light pass through? When and where will you wear it?
A stone in an open pendant at day is different from a ring with a closed setting at night. For the first a strong-colored garnet will be fitting, but for the latter a light-colored sapphire will do much better. If you are not sure about your choices, please ask us.