Since these exceptional gems are fairly rare, they naturally attract the eye of collectors. In some cases all of the gems belonging to a particular variety show the phenomenon, in the way that most moonstone exhibit adularesence. But in other cases the optical effect is located only sometimes in a specific sort of gem, and it's the rare cases that are of special interest to collectors.
The value of a star gemstone depends on the lucidity and distinctness of the star. Often the star must be evaluated using one source light like a penlight ; it is generally awfully tough to see the star under diffused light. Moving the light back and forth should cause the star to move across the stone's surface. The rays of the star should be uniformly distributed and the rays should be comparatively straight. The commonest stars are 4-ray and 6-ray, though 12-ray stars have been seen.
Color is also vital in a star sapphire rings gemstone. Ideally the color should be identical to a non-star sample of the gem, but the presence of rutile in the stone has a tendency to weaken the color. So in the best case there's enough rutile to create a distinct star while minimizing the effect on the color.
Synthetic star sapphires were first produced in the latter 1940's by Linde, a division of Union Carbide. They're now produced in Japan by Nakazumi Earth Crystals. The synthetic gems tend to have perfect color and star so vivid and straight that it would appear painted on the stone.
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