Corundum often contains very fine needles of rutile (TiO2) arranged in intersecting lines in accordance with the symmetry of the crystal. These apparently develop when the stone is formed: as the temperature falls, the TiO;,_. is no longer soluble in the Al2O3 and forms separate crystals. When the needles inside the corundum are particularly numerous, and the stone is cut en cabochon, with its base ("or widest diameter) parallel to the base of the prism, a silk-type reflection is visible in bright light; it is fairly mobile and has the appearance of a six-rayed star, the closer and thicker the rutile needles inside, the more clearly this stands out. When reasonably pronounced, this effect is considered attractive, contributing in the past to the aura of mystery surrounding some gemstones. '
Appearance The most striking phenomenon of rubies and sapphires is the development of the six-ray star, arranged in perfect symmetry, which shifts its center as the stone is moved. It is clearly visible under a single light source such as the sun or a lamp; much less so in diffuse light. lf two or more powerful light sources are set close together, as many stars (their centers not far apart) can be seen in the stone. Each light produces its own star, which is basically a reflection. The effect is usually less pronounced in more transparent stones. The ground color can be ruby red (or an almost grayish, dull red), in which case it is known as a star ruby, or sapphire blue, in which case it is known as a star sapphire, but it can also be blue-gray, smoke gray, or blackish, all of which come under the name of asteria, a generic term also applied to ruby and sapphire. Such stones are invariably given a round or oval cabochon cut. The most highly prized are the ruby-colored (provided they are not the opaque, grayish red of some Indian rubies) and sapphire blue varieties. The others are less valuable, but still sought after, provided the star is clearly visible and they are not too small (3 or 4 carats, at least). Some star stones may weigh 10 carats or more.
Distinctive features The star with its distinctive mobility is characteristic, having six rays, unlike star diopside, forexample, which has four. But to be certain of distinguishing star corundum from the widely divergent (but very few) other gems which can display the phenomenon of asterism, one normally has to measure the physical properties.
Occurrence Rare but magnificent star rubies and sapphires are found in Burma, although most star corundums come from‘Sri Lagka, usually being light blue or gray. Dark asteriae are found in ﬁlysiralia, and dull red, opaque specimens are found in India. Despite being rubies and displaying the phenomenon of asterism, these stones are not very attractive.
Value Star rubies and sapphires of good or even above-
average color areudistinctly valuable, as much so as faceted stones of similar color. The value of the grayish or dark asteriae is lower, though not much, for unusually fine specimens. On the other hand, comparatively small stones of insipid color or with a poorly defined star are worth a great deal less, and the same applies to dull-colored rubies, which often have a broad, smudgy star.
Simulants and synthetics Because of their undoubted attraction, star stones have been imitated_in various ways. Efforts have been made to produce a star by engraving It on the base of a cabochon, or lining the base with a sheet of metal engraved with a six-rayed star. Milky quartz, which exhibits a weak form of the same type of asterism, has also been used, the base of the cabochon being covered “with blue lacquer to give color to the stone and increase the contrast with the star; but the effect is somewhat different from that of natural star stones. In the last few decades, however, some manufacturers of synthetic corundum (using the Verneuil method) have found a way of producing both red and blue star stones with very pronounced stars (more pronounced than the natural versions-), which are not as a rule too transparent and have an attractive, lively color; and these have been a great success in the United States.