Cryptocrystalline Quartz Varieties Occurrence and Use

1. Chalcedony. An amorphous quartz material, translucent with a waxy luster. White, yellowish brown to dark-brown in color. Often maxillary, stalactitic, etc., in structure. Deposited form aqueous solutions and found lining or filling cavities in rocks.
2. Carnelian. A red chalcedony.
3. Chrysoprase. An apple-green chalcedony.
4. Heliotrope or bloodstone. A green chalcedony with small red spots in it
5. Agate. A variegated chalcedony. The different color usually in delicate, fine parallel bands which are commonly curved, sometimes concentric (Fig. B. pl. VI). The color is sometimes strengthened or even changed by artificial means. Some agates have the different colors arranged not in bands but irregularly distributed. Moss agate is a variety in which the variation in color is due to visible impurities, often manganese oxide.
6. Onyx. A banded chalcedony like agate, except the bands are arranged in straight parallel lines.
7. Flint. Something like chalcedony but of dull, often dark color. It breaks with a prominent conchoidal fracture and gives a sharp edge. Used for various implements by early man.
8. Jasper. Opaque quartz, usually colored red from hematite inclusions.

Occurrence. From the result of thermal studies it has been shown that quartz exists in two modifications, called &- and B- quartz is formed between 575° and 870° C. and is characteristic of graphic granite, pegmatites, and porphyries. Above 870° the polymer the polymer known as tridymite is formed and above 1470° that known as cristobalite. Quartz is the most common of minerals. It has been estimated that quartz forms about twelve per cent of the minerals constituting the rocks of the earth’s crust. Occurs as an important constituent of the acid igneous rocks, such as granite, rhyolite, pegmatite, etc. It is a common mineral of sedimentary rocks, forming the chief mineral in sandstone. Occurs largely also in metamorphic rocks, as gneisses and schists, while it forms practically the only mineral of quartzites. Deposited often from solution and forms the most common vein and gangue mineral. In rocks it is associated chiefly with feldspar and muscovite; in veins with practically the entire range of vein minerals. Often carries gold and becomes an important ore of that metal. Occurs in large amount as sand in stream beds and upon the seashore and as a constituent of soils.

Use. Widely used in its various colored forms as ornamental material, as amethyst, rose quartz, cairngorm, cat’s-eye, tiger’s-eye, aventurine, carnelian, agate, onyx, etc. Used for abrading purposes either as quartz sand or as sandpaper. Used in the manufacture of porcelain, of glass, as a wood filler, in paints, scouring soaps, etc. As sand is used in mortars and cements. As quartzite, sandstone, and in its various other rock forms as a building stone, for paving purposes, etc. large amounts of quartz sand are used as an acid flux in certain smelting operations.

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