Rock & Mineral Encyclopedia
Search and identify 1,000+ rocks, minerals, crystals, and gemstones — with properties, formation, colors, hardness, and how to tell them apart.
Chalk
A soft, white, fine-grained limestone made almost entirely of microscopic marine plankton skeletons.
sedimentary
Sphalerite
Zinc sulfide and the chief ore of zinc, prized when transparent for its extreme fire that exceeds diamond.
mineral
Basalt
A fine-grained, dark volcanic rock that erupts as fluid lava and forms most of the ocean floor and many lava plateaus.
igneous
Phonolite
A silica-poor volcanic rock of alkali feldspar and feldspathoids that rings when struck, hence 'clinkstone.'
igneous
Ironstone
An iron-rich sedimentary rock, often heavy and rusty-weathering, historically mined as a major source of iron ore.
sedimentary
Oolite
A limestone made of tiny spherical ooids, resembling fish roe, formed in warm, agitated shallow seas.
sedimentary
Molybdenite
Molybdenite is the primary ore of molybdenum, a soft, greasy, silver-gray sulfide that closely resembles graphite.
mineral
Chromite
Chromite is the only commercial ore of chromium, a black iron-chromium oxide of the spinel group found in mafic igneous rocks.
mineral
Mica Schist
A glittery, strongly foliated rock made mostly of aligned mica flakes that split into thin, shiny sheets.
metamorphic
Purple Opal
A purple-hued common opal, much of it the Mexican "morado" type, valued for even violet color rather than play-of-color.
gemstone
Harlequin Opal
The rarest and most coveted opal play-of-color pattern, showing large, evenly spaced, angular mosaic patches of color.
gemstone
Cipollino Marble
A green-and-white banded metamorphic marble whose wavy mica layers resemble the rings of a sliced onion.
metamorphic
Hawk's Eye
The blue-grey relative of tiger's eye, a chatoyant quartz showing a shifting band of light like a bird of prey's eye.
gemstone
Trachyte
A fine-grained volcanic rock dominated by alkali feldspar, the extrusive equivalent of syenite.
igneous
Patronite
A rare greenish-black vanadium sulfide that was historically one of the world's most important ores of vanadium.
mineral
Fire Opal
A translucent to transparent opal in warm yellow, orange, and red tones, prized for body color rather than play-of-color.
gemstonePantellerite
An iron-rich peralkaline rhyolite, a silica-rich volcanic rock named for the Italian island of Pantelleria.
igneous
Trapiche Emerald
A rare emerald showing a six-spoked star pattern of dark inclusions, named after the spokes of a sugar-mill wheel.
gemstone
Clear Tourmaline
A transparent, water-clear elbaite tourmaline (achroite), the rare colorless and highly transparent form of the tourmaline group.
gemstoneDouble Flow Obsidian
Obsidian formed from two merged lava flows, producing a stone with two distinct bands of sheen or color.
igneous
Spodumene
Spodumene is a lithium aluminum silicate that is both a major lithium ore and the source of gem kunzite and hiddenite.
gemstone
Rainbow Obsidian
A black volcanic glass that reveals concentric rainbow bands of color when cut and polished against the light.
igneous
Water Opal
A transparent, colorless opal that looks like water or jelly, sometimes flashing subtle play-of-color from within.
gemstone
Bastnasite
A rare-earth fluorocarbonate that is one of the world's most important ores of cerium, lanthanum, and other rare earth elements.
mineral