The highest quality Star Sapphires come from Burma, Myanmar, and Ceylon, Sri Lanka. Star Sapphires vary in value according to their intensity, attractiveness of body color and the degree of sharpness of the star. Genuine star sapphires can be recognized by their three-ray, six-point (legs) star design, which are cut into a cabochon. When illuminated with a single light, you will be able to see the star, and it should move across the stone as the light travels across it. This phenomenon is known as "asterism," and it is a result of light reflecting off tiny rutile needles called "silk" along the crystal faces.
Examine The Bottom Of The Gem
Synthetics Star Sapphires often present as "too perfect looking;" they may have a perfect star, excellent color or an impeccably clean stone. Using a magnifying tool, if necessary, look at is the bottom of the gem to see if there is an "L" stamped on the stone. If you find an "L" stamp, it means it is a "Lindy Star" and, therefore, means the gem is synthetic.
If it doesn't have an "L," examine the stone for imperfections within the stone. You are looking for stripes or lines of color that show through the top or an uneveness on the bottom. Most genuine Star Sapphires have one or more of these inherent imperfections.
Using your flashlight, examine the star on your sapphire closely. It would be rare to find a genuine sapphire with a perfect "star." If all the legs are perfectly straight, that would be more indicative of a synthetic sapphire, as genuine sapphires typically have some legs that are not perfectly straight.
Take your flashlight and move it slowly around in a circle over the sapphire. You are looking to see that the star appears to travel around following your light source. If your star appears to stay in the same spot, then it is definitely synthetic.
If your sapphire has passed all the tests performed above, you should still take it to a jeweler or certified gemologist who can carry out a thermal conductivity gem test. This procedure involves using a needle gauge and the dial should jump up to the mark just before diamond to indicate the gem is made of genuine corundum.