In general, star gemstones occur in fewer minerals. In theory, if right conditions were present, almost any mineral could produce asterism.
In a star ruby or sapphire, titanium oxide is exsolved as fine needles of rutile. They orient themselves in three directions, intersecting at 60° and 120° within the same plane producing beautiful stars. The most desirable black star sapphires are those which exhibit the golden star and can be many times more valuable than the more common white-rayed black star sapphires. The asterism of black star sapphire is due to perfectly oriented intergrowth of elongated hematite lamellae. Asterism can be produced in certain types of non-asteriated corundum by moderate heating, if a piece of natural corundum contains enough titanium oxide. Star rubies and sapphires can also be produced by surface diffusion technique, with red and blue being the most common.
Synthetic star corundums by the Verneuil (flame fusion) process are made by adding 0.1-0.3% of titanium oxide to the mixture. The stones are known as Linde stars in the trade and are currently manufactured in several colors, with red and blue being the most common. Identification of synthetic star corundum is not difficult because the needles are usually much smaller than those in the natural stone, and they are more evenly distributed.
Star quartz is sometimes used to imitate star sapphire. The silk usually consists of rutile intersecting in three directions or sillimanite (fibrolite). This may result in a four-rayed star or a six depending upon how the stone is oriented.
Star diopside and star entstatite are two inexpensive stones which are confused with one another. Both display 4-ray stars which meet at almost 90° and one ray is slightly more diffused than the other.
Star beryl is an unusual type. Brown in color and displaying a weak 6-rayed star, it bears a strong resemblance to black star sapphire. The star results from plates of ilmenite and hematite which are oriented in certain positions parallel to the basal plane. These plates also appear to give the stone its dark brown color; if viewed parallel to the plates, clear transparent areas are seen.
The color of star spinel varies from reddish purple to black, with most being quite dark in tone. If the stone is cut so that base is parallel to an octahedron face, a 6-rayed star will be seen; if the stone is cut so that the base is parallel to a cube face, a 4-rayed will be seen. In either case, additional stars will be seen around the girdle. Asterism may also be seen in almandine garnet and moonstone.
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