Rock & Mineral Encyclopedia
Search and identify 1,000+ rocks, minerals, crystals, and gemstones — with properties, formation, colors, hardness, and how to tell them apart.
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
Rock Salt
An evaporite rock of the mineral halite (sodium chloride), the source of common salt, with a distinctive salty taste.
sedimentary
Calc-Silicate Rock
A metamorphic rock of calcium-rich silicate minerals formed from impure limestone or dolomite altered by heat and fluids.
metamorphic
Fossiliferous Limestone
Calcium-carbonate sedimentary rock packed with visible fossils, recording ancient marine life within an easily scratched, fizzing matrix.
sedimentaryDiatomite
A soft, lightweight, chalky sedimentary rock made of the silica shells of microscopic diatoms, prized for its absorbency and filtering ability.
sedimentary
Guano
An accumulated deposit of bird or bat droppings rich in nitrogen and phosphate, historically a prized natural fertilizer.
sedimentary
Turritella Agate
A brown fossiliferous chalcedony packed with spiral freshwater snail shells, technically agatized fossil rock from Wyoming.
sedimentary
Halite
The natural mineral form of table salt, a soft, water-soluble evaporite that forms perfect cubic crystals and tastes salty.
mineral
Coal
A combustible black sedimentary rock formed from ancient plant matter and burned for centuries as a primary fossil fuel.
sedimentary
Cordierite Hornfels
A tough, fine-grained contact-metamorphic rock containing cordierite, often spotted, formed by heat from nearby igneous intrusions.
metamorphic
Shelly Limestone
A limestone packed with visible shells and shell fragments, recording the accumulation of marine invertebrate remains on ancient sea floors.
sedimentary
Dolomite
A calcium-magnesium carbonate mineral and rock similar to limestone but harder and only weakly reactive to acid.
mineral
Gypsum
A very soft sulfate mineral defining Mohs 2, occurring as selenite, satin spar, alabaster, and desert rose, used to make plaster.
mineral
Glaucophane Schist
A blue, high-pressure metamorphic schist rich in glaucophane, the classic rock of subduction zones, also known as blueschist.
metamorphic
Staurolite Schist
A mica schist studded with brown staurolite porphyroblasts, sometimes forming the cross-shaped twins known as fairy stones.
metamorphic
Emerald in Matrix
Natural emerald crystals still embedded in their host rock, prized as mineral specimens that show how the gem grew in place.
gemstoneDiatomaceous Earth
Soft, lightweight siliceous rock made of fossilized diatom shells, valued as a filter, abrasive, and absorbent.
sedimentary
Limestone
A soft carbonate sedimentary rock made mostly of calcite, often packed with marine fossils and prone to forming caves.
sedimentaryGondite
A metamorphic rock made chiefly of manganese-rich spessartine garnet and quartz, formed from ancient manganese-bearing sediments.
metamorphic
Sperrylite
A rare platinum arsenide and the most important platinum-bearing mineral, forming bright metallic cubic crystals.
mineral
Shonkinite
A dark, mafic potassic alkaline rock rich in augite with alkali feldspar and often nepheline, classically forming the base of layered sills.
igneous
Porphyritic Obsidian
Natural volcanic glass speckled with embedded mineral crystals (phenocrysts) such as feldspar or cristobalite that grew before the lava chilled.
igneous
Alnöite
A rare dark ultramafic lamprophyre rich in melilite, biotite and olivine, named for Alnö Island in Sweden.
igneous
Bituminous Shale
A dark, organic-rich shale loaded with kerogen and bitumen that can yield oil and gas, often finely laminated and combustible.
sedimentary