OPAL 



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the majority, however, are cloudy and unsuitable for this purpose. There is a similar 

 occurrence at Crawford, in the State of New York, and some of the brown tourmaline foundl 

 in the limestone of Gouverneur and Newcomb, in New York, is sufficiently clear and free: 

 from fissures to give good cut stones ; but, as a rule, North American tourmaline of brown 

 colour is rarely cut for gems. 



OPAL. 



The widely distributed mineral opal, like the still more frequently occurring quartz, 

 consists mainly of oxide of silicon, that is to say, of silica, but differs from quartz in being 

 not crystalline but amorphous. Besides silica, opal always contains water, the amount 

 varying in different specimens. Various impurities are frequently present, and when in any 

 amount render the stone cloudy and often of a deep colour, so that it is unfit for use as a 

 gem. The variety almost exclusively used for this purpose is that known as the " precious " 

 or " noble " opal ; it is conspicuous amongst all others for the magnificent play of colours 

 produced by the refraction and reflection of light in its colourless substance. This is the 

 variety to which attention in what follows will be mainly directed ; a few of the varieties, 

 which show no play of colour and which are grouped together under the term " common 

 opal," will receive brief mention later on. 



The chemical composition of various kinds of opal used as gems may be seen from the 

 table of analyses given below. The variableness in the amount of water present, and the 

 diversity in the substances present as impurities, should be noticed : 



Since opal is an amorphous substance, it never possesses regular plane-faced boundaries, 

 but occurs usually in rounded nodules, as botryoidal encrustations, as stalactites, and in 

 other forms. There is, of course, a complete absence of cleavage ; the fracture is conchoidal, 

 often typically so. The mineral is moderately brittle, sometimes, indeed, very brittle, when 

 it is easily fractured and broken. It is not very hard ; H = 5J — 6| ; that is to say, it is 

 softer than quartz, so that though opal will scratch glass it is itself scratched by quartz. 

 Because of its brittleness and low degree of hardness it is advisable to protect a gem of 



