Rock & Mineral Encyclopedia

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

Septarian Concretion

A rounded sedimentary nodule cracked internally and filled with veins of yellow calcite, prized for its striking dragon-skin patterning.

A sandstone in which the dominant grains are fragments of pre-existing rocks rather than single minerals, signaling rapid erosion nearby.

A hard, dark, poorly sorted sandstone with a muddy matrix, typically deposited by underwater turbidity currents.

A soft, porous limestone made of loosely cemented shell and coral fragments, used as a coastal building stone.

Calcium-carbonate sedimentary rock packed with visible fossils, recording ancient marine life within an easily scratched, fizzing matrix.

A coarse rock of angular, sharp-edged fragments cemented in a matrix, marking nearby rockfall, faulting, or impact.

Small ironstone concretions from Yowah, Queensland, whose hollow or veined centers hold brilliant precious boulder opal.

An opaque grey-brown common opal forming nodules and concretions, historically called liver opal for its dull brownish color.

Agate that was shattered and naturally re-cemented by silica, creating a mosaic of angular fragments in a quartz matrix.

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

A rare breccia formed by meteorite impact, containing shocked rock fragments and glassy melt blobs welded together.

A limestone packed with visible shells and shell fragments, recording the accumulation of marine invertebrate remains on ancient sea floors.

A jasper made of angular fragments naturally cemented back together, typically showing red and brown pieces in a quartz matrix.

A cohesive fault rock formed by brittle crushing and grinding of rock along a fault zone, with angular fragments in a fine matrix.