Rock consisting mainly of quartz; a variety of this; spec. (a) a very hard sedimentary rock consisting of sandstone in which quartz grains are…

a foliated, crystalline, metamorphic rock composed of alternate layers of mica and quartz and easily split.

= Hornblende-rock or Diabase.

A fine-grained, non-schistose rock composed mainly of quartz, micas, and feldspars and formed by the contact metamorphism of an argillaceous rock.

A compact cryptocrystalline siliceous rock, usually containing some graphite and argillaceous material (clay).

A granular rock largely or entirely composed of hornblende.

Originally: †a fine or smooth-grained sandstone (obsolete). In later use: a sedimentary rock composed of medium-sized particles (commonly defined as…

attributive and in other combinations, as tourmaline crystal, tourmaline granite, tourmaline mink, tourmaline pendant; tourmaline pincette n.,tourma…

attributive and in other combinations, as tourmaline crystal, tourmaline granite, tourmaline mink, tourmaline pendant; tourmaline pincette n.,tourma…

Geology. A rock in which silica, in the form of fine-grained quartz or chalcedony, has replaced some of the original constituents (usually the…

Originally applied to the silicate gangue of certain Archæan iron-ore or other mineral deposits, esp. where these occur in limestone or dolomite; now…

A metamorphic rock formed by alteration of gabbro.

A fully metamorphosed form of a basic rock such as basalt or dolerite.

A green ornamental rock from South Africa.

A coarse-grained crystalline rock consisting of augite and grossularite (or hydrogrossularite), often with prehnite and chlorite, and typically found…

Weathered rock that is largely composed of hydrous aluminium silicates and is highly leached of alkalis.

A rock composed mainly of kaolinite which is commonly found in association with certain coal seams, or a thin band of such a rock (see quots.).

Geology. A greenish quartzite from Greenland, containing fuchsite.