Rock & Mineral Encyclopedia

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

Pseudotachylite

A dark, glassy rock formed when frictional heat from fault movement or impact melts rock along narrow veins.

A prized blue to blue-green copper-aluminium phosphate, often veined with dark matrix, treasured for jewelry across many cultures.

A calcium- and ferrous-iron-rich tourmaline, the iron analogue of uvite, forming dark brown to black crystals in skarns and metamorphic rocks.

A dark, mafic potassic alkaline rock rich in augite with alkali feldspar and often nepheline, classically forming the base of layered sills.

An opaque, iron-rich basaltic volcanic glass formed by the rapid chilling of basalt lava, darker and denser than rhyolitic obsidian.

A dark, glass-rich volcanic rock of olivine and augite phenocrysts set in a feldspar-free glassy groundmass, named from the Kaiserstuhl region.

Idaho's official state gem: a dark almandine garnet showing a four- or rare six-rayed star from oriented rutile inclusions.

A rare alkali-deficient tourmaline whose X crystal site is largely vacant, giving slender dark blue to bluish-black crystals.

Precious opal from Honduras occurring in a dark volcanic matrix, where bright flecks of color flash against a natural black basalt background.

Adirondack Garnet is large, dark-red almandine from New York's Gore Mountain, the world's most famous industrial garnet abrasive source.

A green-and-white banded metamorphic marble whose wavy mica layers resemble the rings of a sliced onion.

A green quartz speckled with shimmering fuchsite mica that produces a glittering aventurescence, popular as an affordable ornamental stone.

A coarse-grained alkali syenite of perthitic feldspar with sodic pyroxene or amphibole and minor nepheline, from Pulaski County, Arkansas.