284. SYSTEMATIC DESCRIPTION OF PRECIOUS STONES 



colour is due to some organic compound. Some authorities have referred it to the presence 

 of a small amount of iron, which has been detected in analysis, while others, relying on 

 Fremy's experiments in the artificial production of rubies, consider it to be due to small 

 quantities of some compound of chromium. 



Asterias (Star-sapphires). There is often to be seen on the basal planes of 

 sapphire crystals a six-rayed star of chatoyant light. This appearance, which is known as 

 asterism, and is often very beautiful, is best displayed by cutting the crystal in which it 

 exists en cabochon, the centre of the curved surface lying in the axis of the crystal. The 

 rays of the star spread out to the margin of the stone, and the movements of the latter are 

 followed to a certain extent by the star, the centre of which is always directed towards the 

 light. A perfect six-rayed star is, however, seen less frequently than a patch of chatoyant 

 light, which may be more or less rounded or elongated in outline, in the latter case being 

 regarded as a single ray of the star. This patch or star of milky, shimmering, and 

 opalescent light is sometimes tinged with a red or blue colour. The rays of the star may 

 be narrow and sharply defined, showing up against the dark surface of the stone like silver 

 threads, or they may be broad and ill-defined, merging imperceptibly into the darker 

 portions of the stone. A sapphire in which the star is sharply defined, is much prized, and 

 is known variously as a star-sapphire, an asteria or star-stone, an asteriated sapphire, or as 

 a sapphire-star-stone. A stone which shows only an irregular patch of opalescent light is 

 known as sappMre-cafs-eye, " oriental g-irasol," or opalescent sapphire. To rubies showing 

 the same appearance corresponding names are applied, namely, star-ruby, asteriated ruby, 

 ruby-star-stone, ruby-cat's-eye, and opalescent ruby. The phenomenon of asterism is not 

 confined entirely to red and blue corundum, but is also occasionally seen in yellow 

 corundum or " oriental topaz." A stone of this kind is, like ruby-cafs-eye, included in the 

 term " oriental giiasol " when it shows an elongated or round patch of opalescent light ; it 

 is then called a " topaz-cafs-eye,'" and when it shows a regular star it is called a " 5tor- 

 topaz.^' 



No exceptional value is attached to asteriated stones ; a fine star-sapphire is about equal 

 in value to an equally fine stone of the ordinary kind, though large star-rubies fetch rather 

 higher prices than ordinary stones ; small ones can be obtained for comparatively little. 

 None of the asteriated varieties of corundum are confined to any special locality, being found 

 wherever precious corundum is found. 



The phenomenon of asterism has been variously explained. Some consider it to be due 

 to the reflection of light from the surface of the twin-lamella, which are present in such 

 crystals in large numbers, and are arranged so that their planes are parallel to the faces of 

 the primitive rhombohedron (Fig. 53 a). The existence of these lamellae is indicated by 

 the striations on the basal planes. These striations are grouped in three sets inclined to 

 each other at angles of 60°, and were considered by Babinet to be the cause of the star of 

 opalescent light. Another and more probable explanation is that it is due to the reflection 

 of light from the surface of a multitude of microscopically small tubular cavities or rifts 

 enclosed in the crystal, and grouped into three sets, each of which is parallel to a face of the 

 hexagonal prism; corundum crystals also enclose minute tabular crystals, consisting of 

 alteration products of the corundum, and arranged, like the rifts, in three sets inclined at 

 60° to each other. The six rays of the star are produced by the total reflection of lio-ht 

 from the surfaces of these tubular cavities, and perhaps also from the surfaces of the tabular 

 crystals. The phenomenon is only to be observed in stones in which there are laro-e 

 numbers of these enclosures, such stones being usually cloudy and having a metallic sheen. 

 Star-stones are never in fact quite clear and transparent throughout their whole mass; 



