The Standard For Star Sapphires
Ideally, the star in a star sapphire should be perfectly centered when viewed from directly above, with each ray of equal length. The rays should be bright, sharp, and clearly defined–not blurry or fuzzy–and they should reach from the crown of the cabochon to the base without interruption.
Star sapphires occur in almost every color seen in transparent sapphires, although yellow, orange, and green star sapphires are very uncommon. The optical phenomenon responsible for star sapphire’s shimmering rays is called asterism, from the Latin word astrum, for “star.” Asterism is caused by small needle-like inclusions of the mineral rutile. Black star sapphires owe their stars and their dark color to included crystals of the shiny mineral hematite.
Clarity in star sapphires is a complicated issue. The ideal star sapphire would be entirely transparent, but the rutile silk that causes the star phenomenon ensures that every star sapphire is to some degree included. Fine star sapphires must have enough silk for a bright, well-defined star, without compromising the stone’s transparency. The best stones are translucent to semi-transparent. Color saturation decreases as opacity increases, so while opaque star sapphires may have very bright, well-defined stars, their color can be dull and unattractive.
How Does Cut Affect A Star Sapphire?
Proper cutting is critical for star sapphires, not only for their overall appearance, but to ensure a well-defined and properly oriented star. Be wary however, if you encounter an absolutely “perfect” star—many man-made star sapphires exist on the market and this is one of their telltale signs.
While most star sapphires remain untreated, they may be subjected to treatments to improve their color or to artificially induce the star phenomenon. Star sapphires can be heated to intensify their color, but this must be done at modest temperatures and with extreme care, so as not to obliterate the star-inducing rutile silk. Since the price of transparent rough has escalated in recent times, most star sapphires are subjected to extreme heat and sold instead as transparent stones. For this reason, fine star sapphires are increasingly more rare and difficult to find.
Where Are They Found?
Star sapphires are found dominantly in Sri Lanka, but also fine blue star sapphires have come from Burma. Black star sapphires have been found in Cambodia and India. Speaking of Burma, rubies also come from this famous source and we have this variety of corundum in our inventory as well.