Corundum often contains very fine needles of rutile arranged in intersecting lines in accordance with the symmetry of the crystal. These apparently developed when the storm is formed: as the temperature falls, the TiO2 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 (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-ray star, the closer and the thicker the rutile needles inside, the more clearly this standout. When reasonably pronounced, this effect is considered attractive, contributing in the past to the aura of mystery surrounding some gemstone.



Appearance:
The most striking phenomenon of rubies and sapphires is the development of the six-ray star, arrange 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 diffused light. If two or more powerful lights sources set close together, as many stars(there 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 and almost grayish, dull red), in which case it is known as a Star-Ruby, or Sapphire Blue, in which case it is known as Star-Sapphire, but it can also be blue-grey, smoke grey, or blackish, all of which come under the name of Australia, a generic term is also applied to ruby and sapphire. Some stones are invariably given a round or oval cabochon cut. The most highly priced are the ruby-color (provided they are not the opaque, grayish red of some Indian rubies) and sapphire blue varieties. The other is less valuable, but still sought after, provided the star is clearly visible and they are too small (three or four carats atleast). Some star stones may weight ten carats or more.



Distinctive Features:
The star with its distinctive mobility is characteristic, having six rays, unlike star diopside, for example, 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 corundum come from Sri Lanka, usually being quite light blue or grey. Dark asteriae are found in Australia, 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 are distinctively valuable, as much so as faceted stones of similar color. The value of 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 the dull-colored rubies, which often have a broad, smudgy star.



Simulants and Synthetic:
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 mettle engraved with the six-rayed star. Milky quartz, which exhibits are weak from the same type of asterism, has also been used, the base of the cabochon been 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 manufactures 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 to transparent and have a attractive, lively color; these have been a great success in the united states.