64 GENERAL CHARACTERS OF PRECIOUS STONES 



the material ; they consist of an unknown substance which is very feebly refiactingr, but it is 

 possible, however, that some are mere vacuities. The whole of the polished surface of such 

 a mineral may reflect the same colour, or different areas of the surface may show different 

 colours. Except with a particular inclination of the light, however, no colour of any kind 

 is seen. On account of this beautiful exhibition of colour, Labrador felspar is more often 

 used for ornamental purposes than is the felspar of Fredriksvarn, the colour of which is less 

 brilliant and variable. 



On certain faces of the minerals hypersthene, bronzite, and diallage, when viewed in a 

 particular direction in reflected light, is to be seen a metallic sheen, uniform in character 

 over the whole surface. These minerals consist of non-metallic substances, and the metallic 

 lustre seen on certain faces is due to the presence of numerous minute plates embedded in 

 the substance of the mineral, parallel to certain directions. When cut and polished with 

 a plane or curved surface parallel to these directions, these minerals are sometimes used as 

 ornamental stones. Hypersthene shows a flne, dark, copper-red reflection ; while in the 

 other minerals grey, yellow, green, and brown colours are predominant. 



A red metallic glittering sheen is also exhibited by avanturine-quartz ; here, however, 

 it is not distributed uniformly over the whole surface, but occurs in numerous small isolated 

 points. These can be seen with the naked eye to be due to small scales of mica enclosed in 

 the quartz ; in the same way, avanturine-felspar, or sun-stone, encloses small plates or 

 scales of hfematite. 



Beautiful eff'ects due to the modification of light are sometimes seen in minerals which 

 possess a more or less pronounced fibrous structure. Such stones when cut with rounded 

 surfaces in the direction of the fibres exhibit a wave of milky light travelling over the 

 surface of the stone as it is moved about. Ordinary cafs-eye, also known as quartz-cat's-eye 

 (Plate XVIII., Figs. 4a and 4b), consists of quartz enclosing numerous fibres of asbestos all 

 arranged in the same direction ; the asbestos may sometimes have been weathered out, in 

 which case the quartz will be penetrated by numerous fine hollow canals. These fibres or 

 canals cause much the same appearance as that seen in adularia and cymophane (oriental 

 cat's-eye), and in this case it also is known as opalescence or chatoyancy. Quartz-cat's- 

 eye may be green, brown, or yellow, and is similar in appearance to the true or oriental 

 cafs-eye. This similarity does not extend to the structure which is the cause of this 

 appearance, for the sheen of quartz-caf s-eye is in reality of the nature of a fine silky lustre, 

 such as is often shown by minerals possessing a fibrous structure, the character of which is 

 modified, however, in the present case, by the nature of the quartz itself. Another variety 

 of fibrous quartz is tiger-eye which is often used for cheap jewellery ; it shows a fine golden 

 reflection, and has a marked tendency to metallic lustre (Plate XVIII., Fig. 5). 



The appearance known as asterism belongs to the same class of phenomena ; it is 

 most frequently seen in raby and sapphire among precious stones, but is not confined to 

 these. When one of the hexagonal crystals of ruby or sapphire (Fig. 53, e-i) has a plane or 

 curved surface cut at the ends, a six-rayed star may be seen by viewing a flame through the 

 stone, or by observing the milky reflection from the surface of the stone. Such stones are 

 known as star-, or asteriated-sapphires or rubies as the case may be, or simply as star-stones 

 or asterias. The effect is produced by reflection of light from a multitude of extremely fine, 

 hollow and long canals. These canals lie in one plane, and are arranged in tfiree directions 

 inclined to one another at 120°. The planes which contain the canals are perpendicular to 

 the principal crystallographic (vertical) axis of the crystal, that is, they are parallel to the 

 plane in which the stone must be cut. According to another view, the star is due to 

 the reflection from numerous twin-lamellae an-anged in three sets. 



