Some of the most interesting members of the gem kingdom are those reflecting bright bands of light that form stars. This optical phenomenon is known as “Asterism”.
The term “Asterism” comes from the Greek word “aster”, which means star. The effect is caused by light reflecting off of bundles of tiny hollow tubes (rutile needles) or fibrous crystals of another mineral inside the gemstone itself.
The sharpness and the size of “Asterism” in gemstones is dependent on the quality and quantity of the rutile needles.
When a properly oriented stone is cut en cabochon, round or oval shape with a domed top and flat bottom, the star is visible. The star may have four to twelve rays depending on the crystal system of the gems. But the six-rayed star effect is most common.
This optical phenomenon is most visible in a direct, single pointed light source, such as an incandescent bulb, fibre optic light, penlight or in direct sunlight.
The most popular of the star gems are star ruby and star sapphire. Other much less commonly found star stones, include chrysoberyl, diopside, enstatite, moonstone and garnet which show four rayed stars.
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.
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. 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.
If it looks to good to be true, it usually is!
We hope this article has given you a bit more of a background on star gemstones, but if you have any additional questions about this or any other jewellery related topics, you can always: “Ask Our Jeweller”.
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