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Chert (/ tʃ ɜːr t /) is a hard, fine-grained sedimentary rock composed of microcrystalline or cryptocrystalline quartz, [1] the mineral form of silicon dioxide (SiO 2). [2] Chert is characteristically of biological origin, but may also occur inorganically as a chemical precipitate or a diagenetic replacement, as in petrified wood.
Cryptocrystalline. Cryptocrystalline is a rock texture made up of such minute crystals that its crystalline nature is only vaguely revealed even microscopically [1] in thin section by transmitted polarized light. Among the sedimentary rocks, chert and flint are cryptocrystalline. Carbonado, a form of diamond, is also cryptocrystalline.
Chalcedony. Chalcedony (/ kælˈsɛdəni / kal-SED-ə-nee, or / ˈkælsəˌdoʊni / KAL-sə-doh-nee) [2] is a cryptocrystalline form of silica, composed of very fine intergrowths of quartz and moganite. [3] These are both silica minerals, but they differ in that quartz has a trigonal crystal structure, while moganite is monoclinic.
Quartz. Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The atoms are linked in a continuous framework of SiO 4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical formula of SiO 2. Quartz is, therefore, classified structurally as a framework silicate mineral ...
Flint, occasionally flintstone, is a sedimentary cryptocrystalline form of the mineral quartz, [1][2] categorized as the variety of chert that occurs in chalk or marly limestone. Historically, flint was widely used to make stone tools and start fires. Flint occurs chiefly as nodules and masses in sedimentary rocks, such as chalks and limestones ...
Jasper, an aggregate of microgranular quartz and/or cryptocrystalline chalcedony and other mineral phases, [1][2] is an opaque, [3] impure variety of silica, usually red, yellow, brown or green in color; and rarely blue. The common red color is due to iron (III) inclusions. Jasper breaks with a smooth surface and is used for ornamentation or as ...
This sets it apart from rock crystal, amethyst, citrine, and the other varieties of crystalline quartz. Other members of the cryptocrystalline silica family include agate, carnelian, and onyx. Unlike many non-transparent silica minerals, it is the color of chrysoprase, rather than any pattern of markings, that makes it desirable.
Greek agate is a name given to pale white to tan colored agate found in the former Greek colony of Sicily as early as 400 BCE. The Greeks used it for making jewelry and beads. Brazilian agate is found as sizable geodes of layered nodules. These occur in brownish tones inter-layered with white and gray.