Rock & Mineral Encyclopedia
Search and identify 1,000+ rocks, minerals, crystals, and gemstones — with properties, formation, colors, hardness, and how to tell them apart.
Coquina
A soft, porous limestone made of loosely cemented shell and coral fragments, used as a coastal building stone.
sedimentary
Limonite
Limonite is an amorphous brown iron oxide ore, the rust-colored material behind ochre pigments and bog iron.
mineral
Limestone
A soft carbonate sedimentary rock made mostly of calcite, often packed with marine fossils and prone to forming caves.
sedimentary
Shelly Limestone
A limestone packed with visible shells and shell fragments, recording the accumulation of marine invertebrate remains on ancient sea floors.
sedimentary
Lithographic Limestone
Extremely fine-grained, even-textured limestone famous for lithographic printing and for preserving exquisite fossils like Archaeopteryx.
sedimentary
Fossiliferous Limestone
Calcium-carbonate sedimentary rock packed with visible fossils, recording ancient marine life within an easily scratched, fizzing matrix.
sedimentary
Oolitic Limestone
Limestone built from tiny rounded ooid grains resembling fish roe, formed in warm, agitated shallow seas.
sedimentary
Crinoidal Limestone
A fossiliferous limestone built largely from the disc-shaped skeletal plates of crinoids, marine animals known as sea lilies.
sedimentary
Chalky Limestone
A soft, fine-grained, porous white limestone made largely of microscopic calcareous plankton skeletons, the rock that forms classic white cliffs.
sedimentary
Micrite
A very fine-grained limestone made of microcrystalline calcite mud, dense and smooth, deposited in calm carbonate settings.
sedimentary
Calcirudite
A coarse-grained limestone built of gravel-sized carbonate clasts, the carbonate equivalent of a conglomerate or breccia.
sedimentary
Coral Rock
A porous limestone built from the calcium carbonate skeletons of corals and reef organisms, the lithified remains of ancient or modern reefs.
sedimentary
Lignite
The lowest rank of coal, a soft brown carbon-rich rock formed from compacted peat, used mainly for electricity generation.
sedimentary
Fire Agate
A rare brown chalcedony containing thin iron-oxide layers that produce flashing, fiery rainbow iridescence like trapped flames.
gemstone
Oolite
A limestone made of tiny spherical ooids, resembling fish roe, formed in warm, agitated shallow seas.
sedimentary
Dolomite
A calcium-magnesium carbonate mineral and rock similar to limestone but harder and only weakly reactive to acid.
mineral
Chalk
A soft, white, fine-grained limestone made almost entirely of microscopic marine plankton skeletons.
sedimentary
Calcilutite
A very fine-grained, mud-sized limestone formed from carbonate mud, smooth and dense with conchoidal fracture.
sedimentary
Golden Healer Quartz
Quartz colored or coated by golden iron oxides such as limonite or goethite, giving a warm sunlit yellow glow.
crystal
Marble
A metamorphosed limestone of interlocking calcite crystals, prized for sculpture and architecture for its workability and polish.
metamorphic
Pisolite
A sedimentary rock built from pea-sized concentric spheres called pisoids, often carbonate but sometimes iron or aluminum-rich.
sedimentary
Bog Iron
A soft, porous iron ore of limonite and goethite that forms in wetlands and bogs, historically the first iron source for many cultures.
sedimentary
Calcarenite
Sand-grained limestone composed of carbonate particles such as shell fragments and ooids cemented into a calcite rock.
sedimentary
Red Sandstone
Iron-stained sandstone whose red color comes from hematite coatings, formed in oxidizing desert, river, and coastal environments.
sedimentary