Rock & Mineral Encyclopedia
Search and identify 1,000+ rocks, minerals, crystals, and gemstones — with properties, formation, colors, hardness, and how to tell them apart.
Shelly Limestone
A limestone packed with visible shells and shell fragments, recording the accumulation of marine invertebrate remains on ancient sea floors.
sedimentary
Coquina
A soft, porous limestone made of loosely cemented shell and coral fragments, used as a coastal building stone.
sedimentary
Calcarenite
Sand-grained limestone composed of carbonate particles such as shell fragments and ooids cemented into a calcite rock.
sedimentaryDiatomaceous Earth
Soft, lightweight siliceous rock made of fossilized diatom shells, valued as a filter, abrasive, and absorbent.
sedimentaryMarl
A soft, earthy sedimentary rock made of a mixture of calcium carbonate and clay, intermediate between limestone and mudstone.
sedimentary
Shell Opal
Fossil shells whose original material has been replaced by opal, preserving ancient marine forms in common or precious opal.
gemstone
Aragonite
A calcium carbonate mineral and polymorph of calcite, aragonite forms distinctive needle clusters, sea shells, and pearls.
mineral
Talc-carbonate Rock
A soft metamorphic rock made of talc and magnesite or dolomite, formed by hydrothermal alteration of ultramafic rocks.
metamorphic
Orange Calcite
A soft, glowing orange variety of calcite colored by iron oxides, popular as tumbled stones and known for fizzing in acid.
mineral
Petrified Wood
Ancient wood whose organic tissue has been replaced by silica, preserving the grain, rings, and structure of the original tree in stone.
sedimentary
Fossiliferous Limestone
Calcium-carbonate sedimentary rock packed with visible fossils, recording ancient marine life within an easily scratched, fizzing matrix.
sedimentary
Cerussite
A dense lead carbonate mineral forming brilliant colorless to white crystals, an important ore of lead and a favorite of collectors.
mineral
Fossil Opal
Fossil material whose original substance has been replaced by opal, preserving ancient shapes in common or play-of-color opal.
gemstone
Siderite
Siderite is an iron carbonate ore, a brown rhombohedral mineral of the calcite group found in sediments and veins.
mineral
Azurite
A deep blue copper carbonate mineral that forms in oxidized copper deposits, often alongside green malachite.
mineral
Malachite
A vivid green copper carbonate mineral famous for swirling concentric bands, used as an ore of copper and an ornamental gemstone.
mineral
Smithsonite
Smithsonite is a zinc carbonate ore famous for glassy botryoidal crusts in blue-green, pink, and yellow hues.
mineral
Calcirudite
A coarse-grained limestone built of gravel-sized carbonate clasts, the carbonate equivalent of a conglomerate or breccia.
sedimentary
Amber
Fossilized tree resin, warm and lightweight, sometimes preserving ancient insects and plant matter inside.
gemstoneOnyx Marble
Translucent banded calcium-carbonate stone deposited in caves and springs, prized for ornamental carvings despite its softness.
sedimentaryJet
A lightweight black organic gemstone formed from fossilized wood under pressure, a type of lignite long used in mourning jewelry.
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
Sinter
A chemical deposit precipitated around hot springs and geysers, either siliceous (geyserite) or calcareous, forming delicate terraces and crusts.
sedimentary
Pisolite
A sedimentary rock built from pea-sized concentric spheres called pisoids, often carbonate but sometimes iron or aluminum-rich.
sedimentary