Rock & Mineral Encyclopedia

Search and identify 1,000+ rocks, minerals, crystals, and gemstones — with properties, formation, colors, hardness, and how to tell them apart.

Tufa

A porous, spongy freshwater limestone that precipitates around springs, streams and lakes, often encrusting plants and moss.

A banded, porous limestone deposited by mineral springs, prized as a warm-toned natural building and tile stone.

A light, porous volcanic rock formed from compacted and cemented ash erupted during explosive eruptions.

Sand-grained limestone composed of carbonate particles such as shell fragments and ooids cemented into a calcite rock.

A soft, earthy sedimentary rock made of a mixture of calcium carbonate and clay, intermediate between limestone and mudstone.

Carbonate-cemented soil crust formed in arid regions where calcium carbonate accumulates and hardens within the regolith.

A soft carbonate sedimentary rock made mostly of calcite, often packed with marine fossils and prone to forming caves.

A soft, fine-grained, porous white limestone made largely of microscopic calcareous plankton skeletons, the rock that forms classic white cliffs.

A soft, white, fine-grained limestone made almost entirely of microscopic marine plankton skeletons.

A chemical deposit precipitated around hot springs and geysers, either siliceous (geyserite) or calcareous, forming delicate terraces and crusts.

A metamorphic rock of calcium-rich silicate minerals formed from impure limestone or dolomite altered by heat and fluids.

A foliated metamorphic rock of calcite mixed with mica, quartz, and calc-silicate minerals, derived from marly sediments.

A rock formed from hot pyroclastic flows, often welded, sometimes containing flattened glass lenses called fiamme.

A yellow-brown alteration material formed when basaltic volcanic glass reacts with water, common in hydrovolcanic tuffs and pillow lavas.

A banded rhyolitic volcanic rock with swirling tan, red, and yellow iron-oxide layers prized as a decorative picture stone.