Rock & Mineral Encyclopedia

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

Anorthoclase

A sodium-rich alkali feldspar of sodic volcanic rocks, sometimes forming large glassy crystals and the blue-flashing feldspar in larvikite.

Precious opal that forms in thin veins within brown ironstone boulders, cut with the host rock left as a natural dark backing.

A mica schist containing bladed blue kyanite crystals, a marker of medium- to high-grade metamorphism of aluminous rocks.

A soft, dark, organic-rich mud deposited in stagnant, oxygen-poor water, a key precursor to oil and gas source rocks.

A black titanium garnet related to andradite and schorlomite, containing tetravalent titanium and ferrous iron, found in skarns and alkaline rocks.

The water-rich end-member of the hydrogrossular series, a soft hydrogarnet found in altered rocks and known from cement chemistry.

A hydrous aluminum garnet (hydrogrossular) intermediate between grossular and katoite, common in altered calcium-rich and serpentinized rocks.

A rare strontium-dominant feldspar, the strontium analogue of paracelsian, found in metamorphosed strontium-rich and manganese-bearing rocks.

A green, vanadium-dominant garnet that forms in vanadium-rich metamorphic and sedimentary rocks, notably in uranium-vanadium districts.

A chromium-dominant tourmaline related to dravite, producing intensely deep green to blackish-green crystals from chromium-rich metamorphic rocks.

A hard zinc-rich member of the spinel group, usually dark blue-green to black, forming octahedral crystals in metamorphic and pegmatitic rocks.

A richly colored red banded agate prized by lapidaries for its deep crimson-to-brown tones reminiscent of pigeon's blood.

A richly saturated green variety of elbaite tourmaline whose color rivals emerald, colored by trace iron, chromium, or vanadium.