PRECIOUS STONES. 140
From the varied origin it will be seen that its associates may be almost any other known mineral.
Chemical Composition: Silicon dioxide, SiO2
Impure forms may contain clay, oxide of iron and many other
substances. Inclusions are common and will be referred to below.
Although Quartz by itself is so infusible, it is readily fused when
mixed with an alkaline carbonate, but this is a cbemical action and
results in the formation of an alkaline silicate (" water-glass ")
which is soluble in water.
A. Pheno-Crystalline or Vitreous Varieties. 1.
Bock Crystal is the ordinary colourless variety; it is always
crystalline and often crystallised. The crystals are usually elongated,
and of sizes varying from a small pin to several feet in length.
Fresange records a crystal from Madagascar measuring 20 feet in
circumference. Dauphine (Bourg d'Oisans) is famous for its groups of
Bock Crystal (Fig. 12). The Alps generally have afforded fine
specimens, and a notable group in the Museum at Naples may be
mentioned. In 1719 a cavity was found at Zinken, in the Bernese
Oberland, from which crystals weighing altogether 50 tons were taken,
and sold for some ±'60,000, some of the crystals weighing up to 8
hundredweight. In Upper Yalais, crystals were found in a cavity which
were of extraordinary size, up to over half a ton in weight. The
Carrara marble quarries have also afforded good specimens. A specimen
in Paris, taken by the French in Italy in 1797, weighs 8 hundredweight
and is 3 feet in diameter. The Caucasus, Siberia, Brazil, and
particularly Japan, have produced fine specimens. In North America,
