436 SYSTEMATIC DESCKIPTIOxN OF PRECIOUS STONES 



both hi situ and also as boulders in glacial deposits. These boulders in the drift can be 

 traced all the way down to Long Island and New Jersey, and they are so numerous in one 

 of the rivers of Lewis County that it has been named Opalescent River. Large quantities 

 of this labradorite are quarried at Keeseville, Essex County, New York, for monumental and 

 building purposes. The mineral is also to be met with at various places in Pennsylvania, 

 Arkansas, and North Carolina, but in no case is the material obtained from these localities 

 cut as a gem, since labradorite from Labrador is not only more easily obtained but exhibits 

 a finer play of colour, and is susceptible of a higher polish than is the case with material from 

 the United States. 



ELJEOLITE. 



Elffiolite is a variety of the mineral species nepheline. It is a silicate of sodium and 

 aluminium, and crystallises in the hexagonal system. Its hardness lies between 5| and 6, 

 and its specific gravity varies between !2"58 and 2"64(. A characteristic feature is the ease 

 with which it is decomposed by hydrochloric acid. Nepheline is found in nature in two 

 forms, which differ widely in external appearance, but possess in common all the features 

 characteristic of the species. Ordinary or " glassy " nepheline takes the form of colourless, 

 or faintly coloured, crystals or irregular grains ; it occurs as a constituent of many of the 

 younger volcanic rocks, drusy cavities in which are often lined with crystals of glassy 

 nepheline. The best crystals are perfectly colourless and transparent, and take the form of 

 hexagonal prisms, which are usually terminated by the basal plane : they are found in the 

 blocks ejected from the old crater of Vesuvius, now represented by Monte Somma. This 

 " glassy " nepheline has none of the characters essential for a gem, and is, therefore, never 

 cut as such. 



The other variety of nepheline occurs as a constituent of ancient plutonic rocks, 

 especially in the elaeolite-syenites of certain districts ; in this case it nearly always takes the 

 form of irregular grains, and seldom occurs in definite crystals. This older nepheUne 

 differs essentially from the younger, glassy variety in the character of its lustre, which is 

 distinctly greasy, hence the name elgeolite (German, Fettstein). Moreover, instead of being 

 transparent and colourless, it is cloudy, or, at most, only translucent, and of a colour 

 ranging from bright bluish-green or brown to tile-red. On mere inspection we would 

 assume elaeolite to be a mineral perfectly distinct from nepheline ; since, however, the two 

 minerals are in complete agreement in such characters as crystalline form, chemical composi- 

 tion, specific gravity, &c., they must be included in the same mineral species. 



The contrast between the external appearance of elaeolite and that of glassy nepheline 

 depends upon the presence in the former of numerous enclosures of microscopically small 

 crystals, some of which may be augite or hornblende, while others belong to other mineral 

 species. Both the colour and the greasy lustre of elseolite is due to these enclosures, the 

 presence of which also gives rise to a wave of soft, milky light reflected with special 

 distinctness from stones cut with a convex, polished surface. The broad band of light 

 crossing the curved surface is very similar to that seen in cymophane and in cat's-eye, and 

 travels over the surface as the stone is moved about before the observer. Stones which 

 combine this appearance with an intense and pure body-colour are often very effective and 

 comparatively valuable, since, though the mineral itself is abundant, specimens of this 



