412 SYSTEMATIC DESCRIPTION OF PRECIOUS STONES 



comparable with that of idocrase. There is also a certain similarity between idocrase and 

 other green stones, such as diopside, epidote, demantoid, &c. : the methods by which these 

 may be distinguished are given in the third part of this book (Tables 13 and 14). 



The idocrase of Vesuvius is found, together with other beautifully crystallised minerals, 

 in the limestone blocks ejected from the old crater of Vesuvius, now represented by Monte 

 Somma. The crystals are attached to the walls of cavities in this metamorphosed limestone, 

 And range in colour from darkest brown to honey-yellow. A cut stone of brown idocrase 

 from Vesuvius is represented in Plate XV., Fig. 3. Some of the pale brown stones found 

 here are much admired ; they are not unlike hyacinth (zircon) in colour, and, indeed, are 

 sometimes mistaken for that stone. There is no difficulty, however, in distinguishing 

 between the two minerals, owing to the density of zircon being so much greater than that 

 of idocrase. The methods by which idocrase may be distinguished from other brown stones, 

 such as smoky-quartz, brown tourmaline, axinite, &c., are given later (Tables 9 and 11). 



The mineral known as xanthite and found at Amity, in Orange County, New York, is 

 nothing more than dark yellowish -brown idocrase ; it is sometimes cut as a gem, but is 

 worn only in the United States. 



For both green and brown idocrase the step-cut or the table cut is employed ; other 

 forms are scarcely ever adopted. In correlation with the limited demand for idocrase as a 

 ^em, we find that both for brown and for green stones quite low prices are asked. 



AXINITE. 



In just a few cases transparent crystals of axinite are cut as gems, usually en cabochon ; 

 tut, as a general rule, crystals of this mineral are only translucent, and therefore unsuitable 

 for this purpose. Axinite has a considerable range of colour ; it may be 

 of a warm clove-brown, sometimes with a noticeable tinge of violet, or, 

 on the other hand, of less pleasing colours containing a large pi-oportion 

 of grey. The dichroism is moderately strong; the images seen in the 

 dichroscope vary in colour between violet-blue, cinnamon-brown, and 

 olive-green. This feature distinguishes axinite from smoky-quartz, the 

 commonest of brown gems, the dichroism of which is very slight ; also 

 form of axinite. ^^°'" brown tourmaline, which, though strongly dichroic, shows different 

 colours in the dichroscope. 

 Chemically, axinite is a silicate of aluminium and calcium, with water and boric acid, 

 and with small quantities of iron and manganese, to the presence of which the colour of 

 the mineral is due. Its composition is represented by the formula : 



HgO.eCaO.BaOg.gAljOg.SSiOa. 



Axinite occurs not infrequently in fine crystals, but most usually in compact masses 

 not suitable for cutting. The crystals belong to the triclinic system, and a frequently 

 occurring form with striated faces is represented in Fig. 74. The crystals are peculiar in 

 that the faces intersect in very acute angles ; owing to this they often take an axe-like 

 form, hence the name of the mineral. There is no distinct cleavage. The mineral is brittle. 

 The hardness (H = 6^ — 7) is approximately that of quartz, and the specific gravity varies 



