In the gemstone world there are a special group of gems known as phenomenal gemstones. These are gems that 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 exhibits 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 (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 a number of other gemstones, such as 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-rayed pattern forming a star. The usual rule of thumb in gemstones is the fewer inclusions the better. However, in the case of asterism, the value is enhanced by desirable inclusions.
The value of a star gemstone depends on the clarity of the star. Generally the star must be evaluated using a single source of 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 relatively straight. The most common stars are 4-rayed and 6-rayed stars, though 12-rayed stars are not unheard of.
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 ideally, there should be enough rutile to create a distinct star whilst maintaining good color.
Although Burma and Sri Lanka are the most important sources for star ruby and star sapphire, Thailand is famous for the black and gold star sapphires found only in Chanthaburi Province. Some star sapphires are produced by using diffusion treatment, whereby 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 1940s 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 stars that are so vivid and straight that they appear to be painted upon the stone.
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