474 SYSTEMATIC DESCRIPTION OF PRECIOUS STONES 



characters. Thirdly, there is the division which includes chalcedony, the special characters 

 of which will be described later. 



The different varieties of quartz which are used for ornamental purposes may, 

 therefore, be classified in the following maimer, and will be dealt with below in the 

 same order : 



A. Ckystalltsf.i) Quartz. 



Rock-crystal. 

 Smo ky- quartz. 

 Amethyst. 

 Citrine. 

 Rose-quartz. 



Prase. 



Sapphire-quartz. 

 Quartz with enclosures. 

 Gat's-eye. 

 Tiger-eye. 



B. Compact Quartz. 



Hornstone (including wood-stone | Jasper. 



and chrysoprase). | Avanturine. 



C. Chalcedony. 



Common chalcedony. 1 Plasma (including heliotrope). 



Carneliau. I Agate (including onyx). 



A. CRYSTALLISED GIXJARTZ. 



KOCK-CBYSTAL. 



Perfectly limpid, colourless, and transparent quartz is known as rock-crystal. It 

 stands out pi'ominontly among all other minerals by reason of its clearness and trans- 

 parency, in which respects it often surpasses even the diamond, although it is not 

 comparable with the latter in lustre or play of colours. An irregular mass of I'ock-crystal 

 at first sight looks very like colourless glass or pure ice, and for this reason it is sometimes 

 called glass-quartz. In ancient times, and indeed even in the Middle Ages, it was thought 

 to be actually ice, which had been frozen so hai-d on the highest peaks of the Alps, where 

 rock-crystal is very abundant, that it could not be thawed again. From this belief arose 

 the use of the word ci-ystal, a term first applied to rock-crystal. 



This variety of quartz occurs very commonly in fine crystals, the prism-faces of which 

 are almost without exception largely developed, so that the habit of the crystals is 

 columnar (Figs. 85a - d). The small faces on the corners, between the prism- and pyramid- 

 faces, sometimes present in this variety, are represented in Figs. 856 and c. Such crystals 

 are, as a rule, attached at one end to the matrix, from which those represented in 

 Figs. 856 - d are broken off, but crystals developed at both ends (Fig. 85a) are by no 

 means uncommon. Crystals of this variety of quartz differ, therefore, in their development 

 from those of common quartz, in which the prism-planes are short, or even altogether 

 absent, and the pyramid-planes only present, while the small faces at the corner's are of very 

 rare occurrence. Rock-crystal often occurs in magnificent groups, like the one represented 

 in Plate XVII. from the neighbourhood of Bourg d'Oisans, in the Dauphine Alps in 

 France. 



There is considerable variation in the size of the crystals. The smallest measure a few 

 millimetres in lengbh and weigh no more than a few milligrams, while the largest, which are 



