454. SYSTEMATIC DESCRiraON OF PRECIOUS STONES 



There is a moderately perfect cleavage parallel to the faces of the rhombic prism. The 

 crystals are brittle, and have a hardness of 6, almost the same as that of felspar ; they are 

 thus easily scratched by quartz but are themselves capable of scratching ordinary window- 

 glass. The specific gravity varies between 3'2 and nearly 3'3, and is higher the greater the 

 amount of iron present. The higher specific gravity is that possessed by the transparent, 

 bottle-green crystals rich in iron from the Tyrol, which furnish most of the material cut 

 as gems. 



The lustre is strongly vitreous and is increased appreciably by polishing. The 

 transparency varies much in degree ; in some crystals it is very nearly perfect, and those 

 which are less perfectly transparent are not cut as gems. Diopside is green in colour, but 

 the depth of shade depends upon the amount of iron present. Crystals which contain 

 practically no iron are almost colourless ; as the amount of ferrous oxide present increases 

 the colour becomes deeper and deeper, and the crystals which are richest in iron are of a 

 fine, deep bottle-green colour. A characteristic feature noticeable even in the most deeply 

 coloured stones is the existence of only a very slight dichroism. 



There are but few localities from which material fit for cutting is obtained. The pale 

 greyish-green crystals which occur in association with hessonite in the Ala valley in 

 Piedmont (Plate XIV., Fig. 7) are worked at Turin, and to a certain extent also at 

 Chamounix, and are worn in rings and other pieces of jewellery, more especially in Italy. 

 Similar crystals of a pale oil-green colour occur at De Kalb, in St. Lawrence County, New 

 York, and are worn to a limited extent in North America. The dark bottle-green crystals 

 from the Schwarzenstein Alp in the Zillerthal, Tyrol, are still finer ; they attain a diameter 

 of 1 inch and a length of 5 inches, and occur attached at one end to chlorite-schist. Some, 

 especially the smaller crystals, are very transparent and of a fine colour, which, however, is 

 frequently not uniform throughout the whole crystal. Very often the attached end of the 

 crystal is green while the free end is almost colourless, but the reverse is never the case. 

 Such crystals were found formerly in some abundance ; the green ends were used for cutting, 

 and the cut stones, like those from Ala, were rather admired, especially in Italy. At the 

 present time they are less frequently found and are worth less than formerly. 



The forms of cutting employed for diopside are those generally used for coloured, 

 transparent stones, namely, the step-cut and the various modifications of the table-stone. 

 In the case of dark -coloured stones the step-cut must not be too deep. 



Diopside may be confused with other green stones or with green glass. It can be 

 distinguished from the latter by the fact that the glass is singly refracting. It differs from 

 emerald in colour and in specific gravity, being much heavier. Chrysolite (olivine) is often 

 very similar to diopside in colour, shows the same feeble dichroism, and has much the same 

 specific gi-avity ; it is appreciably harder, however, than diopside, which is easily scratched 

 by this stone. Green tourmaline, epidote, and alexandrite are readily distinguished from 

 diopside by their strong dichroism ; moreover, the last named is appreciably heavier, and 

 the same is also true of idocrase. Diopside and dioptase can scarcely be mistaken the one 

 for the other, since the difference between them is very marked. 



