Rock & Mineral Encyclopedia

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

Pineapple Opal

A rare opal pseudomorph from White Cliffs, Australia, formed as opal replaced clustered crystals into a pineapple-like shape.

A fine-grained, low-grade metamorphic rock that splits into flat sheets along slaty cleavage, long used for roofing and flooring.

A soft, fine-grained red metamorphosed claystone, sacred to many Native American peoples and carved into ceremonial pipes.

Rhyolite that has been metamorphosed, recrystallizing its silica-rich volcanic material into a tougher felsic metamorphic rock.

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

Obsidian crossed by parallel flow bands of differing color, formed as layers of lava with slightly different compositions froze into glass.

Coarse-grained gabbro that has been metamorphosed, partly recrystallizing into amphibole, plagioclase, and other metamorphic minerals.

A banded agate that diffracts transmitted light into rainbow colors when cut thin and backlit, producing a spectacular iridescence.

Hard surface crust formed when iron oxides cement soil and sediment into a rusty, durable duricrust in tropical and weathered terrains.

A single beryl crystal showing two distinct color zones, such as aquamarine blue grading into morganite pink, within one stone.

A water-free calcium sulfate mineral closely related to gypsum, forming in evaporite deposits and swelling into gypsum when it absorbs water.

A glacier-transported banded agate from the Lake Superior region, colored by iron into rich reds and oranges, and Minnesota's state gemstone.