Rock & Mineral Encyclopedia
Search and identify 1,000+ rocks, minerals, crystals, and gemstones — with properties, formation, colors, hardness, and how to tell them apart.
Alnöite
A rare dark ultramafic lamprophyre rich in melilite, biotite and olivine, named for Alnö Island in Sweden.
igneous
Kentucky Agate
The official state rock of Kentucky, a banded agate famous for striking deep-red and black fortification patterns.
gemstone
Troctolite
A mafic plutonic rock of plagioclase and olivine whose mottled appearance earned it the nickname 'troutstone'.
igneousBanalsite
A rare barium-sodium aluminosilicate of the feldspar group, found chiefly in metamorphosed manganese ore deposits.
mineral
Websterite
A variety of pyroxenite composed of both orthopyroxene and clinopyroxene with little olivine, found in layered intrusions and the mantle.
igneous
Minette
A dark, mica-rich lamprophyre dike rock in which biotite and augite phenocrysts sit in a groundmass dominated by alkali feldspar.
igneous
Oolite
A limestone made of tiny spherical ooids, resembling fish roe, formed in warm, agitated shallow seas.
sedimentary
Calcilutite
A very fine-grained, mud-sized limestone formed from carbonate mud, smooth and dense with conchoidal fracture.
sedimentary
Tactite
A contact-metasomatic calc-silicate rock, essentially a skarn, formed where intrusions react with carbonate rocks and often host ore.
metamorphic
Crocodile Jasper
A deep green-and-black stromatolitic jasper, essentially Kambaba Jasper, with circular eye patterns resembling crocodile skin.
mineral
Pisolite
A sedimentary rock built from pea-sized concentric spheres called pisoids, often carbonate but sometimes iron or aluminum-rich.
sedimentaryPantellerite
An iron-rich peralkaline rhyolite, a silica-rich volcanic rock named for the Italian island of Pantelleria.
igneous
Albitite
A pale rock made almost entirely of the sodium feldspar albite, formed by sodic magmatism or sodium metasomatism.
igneousAppinite
A group of coarse, water-rich plutonic rocks dominated by large hornblende crystals set in feldspar, intermediate between lamprophyre and diorite.
igneous
Tufa
A porous, spongy freshwater limestone that precipitates around springs, streams and lakes, often encrusting plants and moss.
sedimentary
Porcelanite
A hard, fine-grained siliceous rock with a dull porcelain-like texture, intermediate between soft diatomite and dense chert.
sedimentary
Sinter
A chemical deposit precipitated around hot springs and geysers, either siliceous (geyserite) or calcareous, forming delicate terraces and crusts.
sedimentary
Ironstone
An iron-rich sedimentary rock, often heavy and rusty-weathering, historically mined as a major source of iron ore.
sedimentary
Shadow Agate
A banded agate that displays a moving shadow or flash across its surface when tilted under light, caused by closely spaced parallel bands.
gemstone
Comendite
A peralkaline rhyolite, a silica-rich volcanic rock with excess alkalis, named for San Pietro Island's Comende district.
igneousNaujaite
A sodalite-rich agpaitic nepheline syenite with poikilitic texture from the Ilimaussaq complex, packed with blue sodalite, eudialyte and arfvedsonite.
igneous
Limonite
Limonite is an amorphous brown iron oxide ore, the rust-colored material behind ochre pigments and bog iron.
mineral
Picture Jasper
An opaque brown chalcedony whose iron-stained banding mimics deserts, dunes, and distant mountain skylines.
mineral
Variolite
A mafic volcanic rock speckled with pale spherical 'varioles,' typically formed in rapidly chilled basaltic pillow lavas.
igneous