NOTES ON QUARTZ
I shall begin my discriptions with the substance called Quartz. It is pure silica accidentally mixed with minute proportions of metallic oxides from whence the fine colours of this species are derived. By analysis, Rock Crystal yields Silica 99.37% with a trace of alumina, and manganeese. There are many varieties of Quartz, most of which the older Mineralogists discribed as distinct species. Some of these differ considerably in their external characters, others nearly agree. They are sufficiently hard to scratch glass. When compact enough, give fire with the steel, and alone are infusable before the blowpipe. The coloured varieties lose their colour, with carbonate of soda they fuse with effervescence into transparent Glass. Quartz occurs massive and crystallized, also pseudomorphose, spongiform, granular, compact, etc. Its crystals possess double refraction with one positive axis and have the property of polarizing light into a system of single rings first observed by M. Arago. In the massive state, Quartz is a most abundant mineral forming extensive veins in primitive and transition rocks, and being consequently diffused over almost every part of the globe.
There is no specific difference between common Quartz and Rock chrystal. The small and even minute transparent crystals, occuring in almost all metaliferous veins, do not differ from rock crystal in chemical characters. The common form of crystalized Quartz is a six sided prism, terminated by six sided pyramids; the two pyramids joined base to base without an interveneing prism are less frequent. From two pieces rubbed together in the dark, a phosphorescent light is produced. Very few specemens of Quartz or rock crystal occurs in Dauphiné (France) and among the Alps, gigantic crystals have been found in Madagascar, & Brazil. The most beautifull specemens found in U. States is in Herkemer County N. York. Some of the crystals contain air [and/or] some water, which move to and fro: by changing the position of the crystal. Crystals of Quartz are occasionally found enclosing foreign crystalized substances as shorl, asbestous, actynolite, titanium etc. The following sub-species have been distinguished:
Amethyst: It yields 97.50% Silica with a trace of oxide of Iron, Alumina & manganeese. Amethyst chiefly differs from Common Quartz in its colour, which is purplish violet, supposed to be derived from a minute proportion of Iron and Manganeese which it contains. It becomes white by a long exposure to heat.
Aventurine: A variety of quartz rock including small laminae of Mica, which when polished, presents a shining spangled-like appearance. The most common colour of the base is brown or redish brown, enclosing spangles of a gold colour, as in the variety from Cape de Gatte Spain.
Prase: This variety possesses a dark leek green colour (whence its name from the Greek). It only occurs massive, it owes its fine colour to Actmolite: it is found in Saxony & U States.
Milk Quartz: It occurs massive, and is only distinguished by its colour. As its name denotes, it presents a milky aspect.
Rose Quartz: It is supposed to derive its colour from a minute admixture with manganeese, and is found in Bavaria and the United States. The best are brought from India, Siberia, Ceylon, and Persia. The finest I ever saw are from North Carolina and Cape Plumedon at the head of Bay of Fundy.
Yellow and Brown Quartz: Fine specemens are found in Cairngorum, and hence has been called by that name.
Ferruginous Quartz: This variety is opaque of several shades of yellow and red, occurs massive and crystalized. It consists chiefly of Silica and 5% of Iron.
Radiated Quartz: It occurs in crystals which radiate from a point.
Fibrous Quartz: It is produced when the composition presents their Columnar particles. The Cats-eye is a variety of this, with thin filaments of Asbestous, which when the stone is cut on ‘cabochon’ as the French call it, presents a peculiar opalescent, or chatoyant streak of light. It is usually of a greyish or greenish colour, it contains small portions of Alumina and Lime, it is translucent and used as an ornamental stone, it is brought from Ceylon and Malabar.
Spongiform Quartz: It occurs in beds of Flint in chalk near Paris, it presents a spongy or porous appearance, consists of numerous minute whitish crystals, and as its name indicates, possesses the property of swimming on water, at least ‘till the air contained in its numerous cavities is displaced.