QUARTZ. 



773 



stances which consist of mixtures of quartz with 

 other species; and the difficulty of forming a cor- 

 rect idea of this group of minerals has been enhan- 

 ced by their being separated into several species 

 and sub-species, as has been done in certain trea- 

 tises on mineralogy. We shall endeavour, in the 

 first place, to take a scientific view of quartz; after 

 which we shall point out those varieties which have, 

 from various causes, obtained distinct appellations, 

 together with the mixtures of quartz with other 

 minerals, usually considered as varieties of the spe- 

 cies; and, finally, the varieties which are employed 

 in the arts. 



The crystals of quartz are, for the most part, re- 

 gular six-sided prisms, terminated at one or both of 

 their extremities by six-sided pyramids, the faces 

 of which correspond to the sides of the prism, and 

 meet under an angle of 141 40'. It sometimes 

 happens that the prism intervening between the two 

 pyramids is very short, or even extinct; in which 

 case the form becomes the dodecahedron, with isos- 

 celes triangular faces. In the six-sided prism with 

 pyramidal terminations, we often observe the solid 

 angles, situated between the prism and the pyra- 

 mid, replaced by rhomboidal planes. The alter- 

 nate faces of the prism are striated horizontally. 

 The primary form is a rhomboid of 94 15' and 85 

 45', parallel to whose planes cleavages may be ob- 

 tained, as also to the planes of the dodecahedron, 

 with isosceles triangular faces, which might also be 

 regarded as the primary form of the species; but 

 the former figure is preferred on account of its sim- 

 plicity. Lustre vitreous, inclining, in some varie- 

 ties, to resinous; colour white prevalent; among 

 the brightest colours are violet-blue, rose-red, clove- 

 brown and apple-green. Dark brown and green 

 colours are generally owing to foreign admixtures. 

 Streak white; transparent, translucent, frequently 

 opaque, but never, perhaps, unless occasioned by 

 other minerals ; hardness between feldspar and spi- 

 nelle; specific gravity 2.69. The crystals often pre- 

 sent macles, and sometimes we observe larger cry- 

 stals, apparently made up of innumerable smaller 

 ones, all of which are so aggregated that their simi- 

 lar faces coincide as respects direction. Implanted 

 globules, reniform and stalactitic shapes, are other 

 modes of occurrence with compound varieties of 

 this species. These have smooth, granulated and 

 drusy surfaces; composition (mechanical) columnar, 

 often impalpable, and frequently a second time com- 

 posed into granular or curved lamellar masses. The 

 massive varieties present a granular or columnar, 

 and often an impalpable composition, in which case 

 the fracture becomes conchoidal and splintery. 

 Sometimes a second composition produces indistinct 

 granular or thick lamellar masses. Certain very 

 thin columnar compositions, if cut en cabuchon, 

 parallel to the fibres, show an opalescent light. We 

 have pseudomorphous crystals, also.of this species, in 

 the shape of cubes, octahedrons, and various other 

 forms, derived from fluor, calcareous spar, gypsum, 

 &c., besides which, quartz occurs in globular and 

 tuberose masses, formed in vesicular cavities, and 

 also in plates and pebbles. 



The principal varieties of quartz, which are 

 known by distinct names, are the following : l.rock 

 crystal; 2. smoky quartz; 3. yellow quartz; 4. ame- 

 thyst; 5. siderite, or blue quartz; 6. rose quartz; 1. 

 milky quartz; 8. irised quartz; 9. common quartz; 

 10. fat (greasy] quartz; 11. flint; 12. hornstone; l.'i. 

 Lydian stone; 14. floatstone (swimming stone); 15. 

 fibrous quartz; 16. radiating quartz; 17. chalcedony; 

 18. cornelian; 19. chrysoprase; 20. agate. Rock 

 crystal is applied to the transparent and colourless 

 crystals, and more particularly to those of a large 



size. Smoky quartz consists of those crystals and 

 crystalline masses which are translucent and of a 

 brown colour. Yellow quartz, sometimes called, 

 also, Bohemian or Scottish topaz, is transparent, 

 and of various shades of yellow. Amethyst, also 

 in crystals, and for the most part nearly transparent, 

 is of every shade of violet. Siderite, or blue quartz, 

 is never in regular crystals, but usually compact, 

 and of an azure-blue colour. Rose quartz is con- 

 fined to translucent massive varieties, of a delicate 

 rose-red colour. Milky quartz is also massive, hav- 

 ing an uneven fracture, is translucent, and of a 

 milk-white colour. Irised quartz embraces such 

 crystallized varieties as exhibit in patches, at or 

 beneath the surface, the colours of the rainbow. 

 Common quartz differs from milky quartz simply in 

 being destitute of the milky whiteness of that vari- 

 ety, or in having an inferior degree of whiteness, 

 and more of a vitreous lustre. Fat or greasy quartz 

 differs from common quartz merely in lustre, which, 

 instead of being vitreous, has the appearance of 

 having been immersed in oil. Flint has a more 

 compact texture than common quartz, is dull, 

 only translucent on the edges, of a brownish col- 

 our, and breaks with a conchoidal fracture. Horn- 

 stone closely resembles flint, from which it can 

 scarcely be distinguished, except in its conchoidal 

 fracture, which is much less distinct. When re- 

 placing the fibres of wood, it is called woodslone. 

 Lydian stone, sometimes called flinty slate, differs 

 from flint chiefly in having a darker colour, less 

 translucency, and a fracture somewhat slaty ; when 

 black, it is often called bassanite. Floatstone con- 

 sists of a delicate tissue of minute crystals, visible 

 only under a powerful magnifier. Owing to the 

 cavities it contains, it sometimes will float on water. 

 Fibrous quartz consists of those varieties which are 

 in distinct, parallel concretions. Radiating quartz 

 is like fibrous quartz, except that the fibres diverge 

 from a common centre, and resemble the radii of a 

 circle, instead of being parallel. Chalcedony in- 

 cludes those varieties of radiating quartz, where the 

 thickness of the individuals becomes so much di- 

 minished as to render them nearly or altogether 

 impalpable; its masses generally offer a mammillary 

 or stalactitical surface, and the lustre on freshly- 

 broken surfaces is feeble. Carnelian differs from 

 chalcedony merely in having a blood-red colour. 

 Chrysoprase also resembles chalcedony in composi- 

 tion, excepting that the individuals of which it is 

 made up, appear to be grains instead of fibres ; its 

 colour is apple-green, from the oxide of nickel. 

 Agate implies the occurrence of two or more of the 

 above varieties, existing together in intimate union; 

 or it may be applied to a single variety, provided it 

 offers different colours in the same specimens. 



Those substances which have formerly been in- 

 cluded under quartz, and which are mixtures only 

 of this mineral with other species, are the following: 

 1. oaf s eye; 2. avanturine ; 3. prase ; 4. plasma; 

 5. heliotrope; Q. iron-flint ; 7. Compostella hya- 

 cinth ; S.jasper (red, brown, striped and porcelain}; 

 Q. jasper agate ; 10. Mocha stone; 11. Venus-hair 

 agate. Cat's eye consists of transparent quartz 

 traversed by exceedingly delicate parallel fibres of 

 asbestus ; when cut en cabuchon, it exhibits, as its 

 position is altered to the eye, a peculiar play of 

 light upon its surface, resembling the changeability 

 of light seen in the eye of a cat. Avanturine is 

 quartz penetrated by small golden-coloured span- 

 gles of mica. Prase is quartz penetrated by green 

 fibres of hornblende, which are often so small as to 

 escape detection by the naked eye, and their pres- 

 ence is only discoverable from the green colour they 

 impart. I'lasma is a variety of chalcedony, coloured 



