Rock & Mineral Encyclopedia
Search and identify 1,000+ rocks, minerals, crystals, and gemstones — with properties, formation, colors, hardness, and how to tell them apart.
Goethite
Goethite is a common brown iron oxyhydroxide, the main crystalline component of limonite and rust, with shimmering botryoidal forms.
mineral
Ironstone
An iron-rich sedimentary rock, often heavy and rusty-weathering, historically mined as a major source of iron ore.
sedimentary
Limonite
Limonite is an amorphous brown iron oxide ore, the rust-colored material behind ochre pigments and bog iron.
mineral
Golden Healer Quartz
Quartz colored or coated by golden iron oxides such as limonite or goethite, giving a warm sunlit yellow glow.
crystal
Bog Iron
A soft, porous iron ore of limonite and goethite that forms in wetlands and bogs, historically the first iron source for many cultures.
sedimentary
Boulder Opal
Precious opal that forms in thin veins within brown ironstone boulders, cut with the host rock left as a natural dark backing.
gemstone
Fire Agate
A rare brown chalcedony containing thin iron-oxide layers that produce flashing, fiery rainbow iridescence like trapped flames.
gemstone
Siderite
Siderite is an iron carbonate ore, a brown rhombohedral mineral of the calcite group found in sediments and veins.
mineral
Lignite
The lowest rank of coal, a soft brown carbon-rich rock formed from compacted peat, used mainly for electricity generation.
sedimentary
Koroit Opal
Boulder opal from the Koroit field in Queensland, famous for intricate ironstone matrix patterns laced with colorful precious opal.
gemstone
Yowah Nut Opal
Small ironstone concretions from Yowah, Queensland, whose hollow or veined centers hold brilliant precious boulder opal.
gemstone
Ferricrete
Hard surface crust formed when iron oxides cement soil and sediment into a rusty, durable duricrust in tropical and weathered terrains.
sedimentary
Jaspillite
A banded, metamorphosed iron formation in which bright red jasper alternates with silvery hematite or magnetite layers.
metamorphic
Banded Iron Formation
Ancient chemically deposited rock of alternating iron-oxide and silica bands recording Earth's early oxygenation and a major iron ore source.
sedimentary
Tangerine Quartz
Clear quartz coated with orange-red hematite, giving points a vivid tangerine color, mainly from Brazil.
crystal
Quilpie Opal
Boulder opal from the Quilpie district of Queensland, Australia, with bright color set in dark ironstone matrix.
gemstoneRed Sandstone
Iron-stained sandstone whose red color comes from hematite coatings, formed in oxidizing desert, river, and coastal environments.
sedimentaryJet
A lightweight black organic gemstone formed from fossilized wood under pressure, a type of lignite long used in mourning jewelry.
sedimentaryStone Canyon Jasper
A warm-toned brecciated jasper from central California known for swirling browns, golds, and creams broken by darker seams.
gemstone
Coal
A combustible black sedimentary rock formed from ancient plant matter and burned for centuries as a primary fossil fuel.
sedimentary