To physicists quartz is one of the most interesting of minerals because of its power of rotating, to an extent depending upon the thickness of the section, the plane of polarization of a beam of light traversing it in a direction parallel to the prism edge. It appears, moreover, from a study of the pyro-electric and general physical characters, that its molecular structure has a helical arrangement, which, like all screws, may have a right- or left-handed character. Amethyst is, in fact, invariably composed of separate twin individuals, alternately right- and left-handed; in some remarkable crystals the section at right angles to the prism edge is composed of triangular sectors, alternately of different hands and of different tints—purple and white. To the twinning is due the rippled fracture and the feathery inclusions so characteristic of amethyst.
Besides its use for ornamental purposes, quartz finds a place as the material for lenses intended for delicate photographic work, because its transparency to the ultra-violet light is so much greater than that of glass. Spectacle lenses made of it are in demand, because they are not liable to scratches, and retain, therefore, their polish indefinitely. When fused in the oxyhydrogen flame, quartz becomes a silica glass, of specific gravity 2·2 and hardness 5 on Mohs’s scale, which has proved of great service for laboratory ware, because it withstands sudden and unequal heating without any danger of fracture; it has also in fine threads been invaluable for delicate torsion work, because it acquires not the smallest amount of permanent twist, in this respect being superior to the finest silk threads.
Clear rock-crystal fetches little more than the cost of the cutting; citrine and amethyst are worth from 1s. to 5s. a carat, depending upon the quality and size of the stone; smoky-quartz is practically valueless; rose-quartz realizes less than 1s. a carat; and the value of cat’s-eye is also small—only 1s. to 2s. 6d. a carat. Tiger’s-eye at one time commanded as much as 25s. a carat, but the supply exceeded the demand, with the consequent collapse in the price.
Beautiful, brilliant, and limpid rock-crystal is found in various parts of the world: in the Swiss Alps, at Bourg d’Oisans in the Dauphiné Alps, France, in the famous Carrara marble, in the Marmaros Comitat of Hungary, and in the United States, Brazil, Madagascar, and Japan. Small lustrous stones, known in their localities as ‘Isle of Wight,’ ‘Cornish,’ or ‘Bristol diamonds,’ are found in our own country. Brazil supplies stones out of which have been cut the clear balls used in crystal-gazing. The finest amethysts come from Brazil—especially the State of Rio Grande do Sul—and from Uruguay, India, and the gem-gravels of Ceylon; good stones also occur at Ekaterinburg, in the Ural Mountains. A splendid Brazilian amethyst, weighing 334 carats, and two Russian stones—one hexagonal in contour, weighing 88 carats, and the other, a deep purple in colour with a circular table, weighing 73 carats—are exhibited in the British Museum (Natural History). Cairngorm is known from the place of that name in Banffshire, Scotland, whence fine specimens have emanated; it is a gem much valued in that country. Fine cairngorm has also originated from Pike’s Peak, Colorado. Splendid yellow stones have had their birth in the States of Minas Geraes, São Paulo, and Goyãz, of Brazil—especially in the last. The fine Spanish smoky-quartz, which, as already stated, turns yellow on heating, comes from Hinojosa, in the Province of Cordova. The delicate rose-quartz is known at Bodenmais in Bavaria, Paris in Maine, United States, and Ekaterinburg in the Ural Mountains. The finest cat’s-eyes are found in India and Ceylon, and are high in favour with the natives. Greenish stones of an inferior quality are brought from the Fichtelgebirge in Bavaria, and are sold as ‘Hungarian cat’s-eyes,’ despite the fact that no such stone occurs in Hungary—another instance of jewellers’ disdain for accuracy. Tiger’s-eye occurs in considerable quantity in the neighbourhood of Griquatown, Griqualand West, South Africa. A silicified crocidolite, in which the blue colour is retained, comes also from Salzburg, and is known as sapphire- or azure-quartz, or siderite.
Certain of the pebbles found on the seashore of our coasts, especially off the Isle of Wight and North Wales, cut into attractive, clear stones, more or less yellow in colour; but examples suitable for the purpose are not so numerous as might be supposed, and do not reward any casual search. Les affaires sont les affaires. The local lapidary, instead of explaining that the pebbles brought to him are not worth cutting, finds it more convenient and profitable to substitute for them other, inferior and badly cut, stones, bought by the gross, or even paste stones; the customer, on the other hand, is contented with a pretty bauble, and is not grateful for the information that it might have been obtained for a fraction of the sum paid.
CHAPTER XXIX
CHALCEDONY, AGATE, ETC.
CHALCEDONY and agate, and their endless varieties, are composed mainly of silica, but the separate individual crystals are so small as to be invisible to the unaided eyesight, and occasionally are so extremely minute that the structure is almost amorphous. The colour and appearance vary greatly, depending upon the impurities contained in the stone, and, since both have been made a criterion for differentiation of types, a host of names have come into use, none of which are susceptible of strict definition. On the whole, these stones may be divided into two groups: chalcedony, in which the structure is concretionary and the colour comparatively uniform, and agate, in which the arrangement takes the form of bands, varying greatly in tint and colour.
The refraction, though double in the individual, is irregular over the stone as a whole, and the indices approximate to 1·550. The specific gravity ranges from 2·62 to 2·64, depending upon the impurities present. The degree of hardness is about the same as that of quartz, namely, 7 on Mohs’s scale. All kinds are more or less porous, and stones of a dull colour are therefore artificially tinted after being worked.