About Star Gemstones
In the gemstone world there are a special group of gems known as phenomenal gemstones. These are gems which exhibit special optical phenomena, including asterism (the star effect), chatoyancy (the cat's eye effect), adularescence (shimmering light) and iridescence (the rainbow effect).
Since these phenomenal gems are fairly rare, they naturally attract the attention of collectors. In some cases all or most of the gems belonging to a particular variety display the phenomenon, in the way that most moonstone exhibit adularesence. But in other cases the optical effect is found only occasionally in a particular type of gem, and it is the rare cases that are of special interest to collectors.
Asterism or the star effect is an example of an optical effect found very rarely in a small number of gem types cut as cabochons. The most famous examples are star sapphire and star ruby. But asterism may also be exhibited by moonstone, quartz, spinel, citrine, diopside, emerald, garnet and chrysoberyl.
The reason that asterism is so rare is that it requires the gem to have inclusions of rutile needles, and for the needles to be aligned in such a way as to reflect light in a multi-ray pattern forming a star. The usual rule of thumb in gemstones is the fewer inclusions the better. But this is a case where the value is enhanced by the right sort of inclusions.
The value of a star gemstone depends on the clarity and distinctness of the star. Generally the star must be evaluated using a single source light such as a penlight; it is usually very difficult 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 evenly distributed and the rays should be relatively straight. The most common stars are 4-ray and 6-ray, though 12-ray stars have been seen.
Color is also very important in a star gemstone. Ideally the color should be equivalent to a non-star specimen of the gem, but the presence of rutile in the stone tends to weaken the color. So in the best case there is enough rutile to create a distinct star while minimizing the effect on the color.
Burma and Sri Lanka are the most important sources for star ruby and star sapphire, though Thailand is famous for the black and gold star sapphires found only in Chanthaburi province. Some star sapphires are produced using a diffusion treatment, where the sapphire is heated with a coating of titanium dioxide which diffuses into the corundum and creates rutile needles.
Synthetic star sapphires were first produced in the late 1940's by Linde, a division of Union Carbide. They are currently produced in Japan by Nakazumi Earth Crystals. The synthetic gems tend to have perfect color and star so vivid and straight that it appears painted on the stone.