Rock & Mineral Encyclopedia
Search and identify 1,000+ rocks, minerals, crystals, and gemstones — with properties, formation, colors, hardness, and how to tell them apart.
Spherulitic Obsidian
Obsidian containing spherulites — small radiating spheres of feldspar and cristobalite that crystallized within the cooling volcanic glass.
igneous
Mintabie Opal
Precious opal from the Mintabie field in South Australia, known for hard, bright crystal opal and some dark-bodied stones.
gemstone
Aventurine
A translucent quartz speckled with glittery mineral inclusions that produce a shimmering aventurescence, most often green.
crystal
Breccia
A coarse rock of angular, sharp-edged fragments cemented in a matrix, marking nearby rockfall, faulting, or impact.
sedimentary
Scoria
A dark, highly vesicular volcanic rock full of gas bubbles, denser than pumice, common as red or black lava rock.
igneous
Eulysite
A rare, dense iron-rich metamorphic rock composed of fayalite, iron pyroxene, and almandine garnet.
metamorphic
Sandstone
A clastic sedimentary rock made of cemented sand grains, often quartz, recording ancient beaches, deserts, and rivers.
sedimentary
Iceland Spar
A transparent, optical-grade variety of calcite famous for strong double refraction, splitting images and light into two rays.
mineral
Green Aventurine
A green quartz speckled with shimmering fuchsite mica that produces a glittering aventurescence, popular as an affordable ornamental stone.
mineral
Coquina
A soft, porous limestone made of loosely cemented shell and coral fragments, used as a coastal building stone.
sedimentary
Basalt
A fine-grained, dark volcanic rock that erupts as fluid lava and forms most of the ocean floor and many lava plateaus.
igneous
Diorite
A coarse-grained intrusive rock with a distinctive salt-and-pepper look, the plutonic equivalent of andesite.
igneous
Needle Tourmaline
Fine acicular (needle-like) tourmaline crystals, often black schorl, frequently seen as slender inclusions within clear quartz.
mineral
Brazilian Opal
Precious opal from Brazil, especially the Pedro II area of Piaui, known for bright, often stable crystal and white opal.
gemstone
Calcite
An extremely common calcium carbonate mineral that comes in nearly every color and shows strong double refraction in clear crystals.
mineral
Melteigite
A dark, pyroxene-dominated plutonic rock at the mafic end of the ijolite series, made mainly of aegirine-augite with subordinate nepheline.
igneous
Ijolite
A coarse-grained, feldspar-free plutonic rock composed mainly of nepheline and sodic pyroxene, the intrusive equivalent of nephelinite.
igneous
Hornfels
A tough, fine-grained, non-foliated rock formed by the intense heat of nearby magma baking surrounding rock at contact zones.
metamorphic
Prasiolite
A pale green variety of quartz, usually created by heat-treating amethyst, often marketed as green amethyst.
gemstone
Skarn
A calc-silicate rock formed by chemical exchange between magma and carbonate rock, often rich in garnet and economically important ore minerals.
metamorphic
Pisolite
A sedimentary rock built from pea-sized concentric spheres called pisoids, often carbonate but sometimes iron or aluminum-rich.
sedimentary
Radiolarite
A hard, fine-grained siliceous rock built from the microscopic silica skeletons of radiolarians, often forming colorful ribbon-banded cherts.
sedimentary
Adinole
A fine-grained, sodium-rich contact-metasomatic rock formed where shale is albitized next to intruding diabase or spilite.
metamorphic
Mica Schist
A glittery, strongly foliated rock made mostly of aligned mica flakes that split into thin, shiny sheets.
metamorphic