A comprehensive A-Z guide to 48 rocks. Every entry is backed by real geological science. Every rock was included without being consulted.
A dark, dense metamorphic rock rich in amphibole minerals (especially hornblende) and plagioclase feldspar. Forms under medium to high-grade metamorphism of mafic igneous rocks like basalt and gabbro. Common in continental shield regions and mountain belts worldwide.
Named after the Andes Mountains, andesite is an intermediate volcanic rock common at subduction zones. Fine-grained with a composition between basalt and rhyolite. It's the signature rock of continental volcanic arcs and makes up a significant portion of the Earth's volcanic output.
The highest grade of coal, containing 86-97% carbon. Formed from bituminous coal through additional metamorphism. Burns hotter and cleaner than other coals, with a distinctive semi-metallic luster. Most anthracite formed during the Carboniferous period from ancient swamp forests.
A coarse-grained sandstone containing at least 25% feldspar, typically pink or reddish from potassium feldspar. Indicates rapid erosion of a nearby granitic source rock, deposited before the feldspar could weather to clay. Often found near mountain fronts and fault scarps.
The most abundant volcanic rock on Earth, forming the entire ocean floor and volcanic islands. Fine-grained and dark, composed mainly of plagioclase and pyroxene. Erupts at temperatures of 1,000-1,200°C. More basalt has been produced than all other volcanic rocks combined.
Forms exclusively at high pressures and relatively low temperatures in subduction zones, where oceanic crust is dragged beneath continental plates. The distinctive blue color comes from the mineral glaucophane. One of the rarest metamorphic rock types — its existence is direct evidence of plate tectonics.
Composed of angular rock fragments cemented together in a fine-grained matrix. Unlike conglomerate, the fragments are sharp and angular, meaning they weren't transported far from their source. Can form from landslides, fault zones, volcanic explosions, or meteorite impacts.
A soft, white, porous limestone made almost entirely of coccolithophore shells — microscopic marine algae. The White Cliffs of Dover are pure chalk, deposited in a warm, shallow sea during the Late Cretaceous. The word 'Cretaceous' literally comes from the Latin for chalk.
An extremely hard sedimentary rock made of microcrystalline quartz (SiO2). Includes flint, jasper, and agate as varieties. Fractures with sharp conchoidal edges, making it humanity's first cutting tool material. Often forms as nodules in limestone.
An organic sedimentary rock formed from compressed ancient plant material, primarily from Carboniferous period swamp forests (359-299 million years ago). Ranks from lignite to bituminous based on carbon content and heat value. Powers much of modern civilization.
Composed of rounded pebbles, cobbles, or boulders cemented in a sandy or silty matrix. The rounded fragments indicate significant transport by water, which tumbled sharp edges smooth. Often forms in river channels, alluvial fans, or beach environments.
A bioclastic limestone made almost entirely of loosely cemented shell fragments — clams, snails, and other marine organisms. The shells are typically visible to the naked eye. Found in warm, shallow marine environments. Used as building stone in coastal fortifications.
An intermediate to felsic volcanic rock compositionally between andesite and rhyolite. Often associated with explosive eruptions at subduction zone volcanoes. Mount St. Helens' 1980 eruption produced dacite lava domes. Contains plagioclase, quartz, and sometimes biotite or hornblende.
Also called dolostone, composed primarily of the mineral dolomite (CaMg(CO3)2). Looks similar to limestone but contains magnesium. Forms through replacement of calcium by magnesium in existing limestone. The Dolomite mountains in Italy are named after it (or vice versa).
An ultramafic rock composed of over 90% olivine. Found in the Earth's upper mantle and in ophiolite complexes where mantle rock has been thrust onto continental crust. Dense, green, and rich in magnesium and iron. One of the deepest-origin rocks accessible at the surface.
A strikingly beautiful rock formed under extreme pressure and temperature deep in subduction zones (45+ km depth). Composed of red garnet and green omphacite (a pyroxene), giving it a distinctive Christmas-colored appearance. Among the densest of all crustal rocks.
A general term for any light-colored, fine-grained volcanic or hypabyssal rock rich in silica and alkali feldspar. Includes many rhyolites and trachytes when their exact composition hasn't been determined in the field. Common in volcanic dikes and sills.
A hard, dark variety of chert found as nodules in chalk and limestone formations. Composed of microcrystalline quartz. Produces sparks when struck against steel — a property that gave humanity fire-starting capability and powered flintlock firearms for centuries. Flint tools date to 2.6 million years ago.
The coarse-grained plutonic equivalent of basalt. Forms deep beneath mid-ocean ridges and in large igneous intrusions. Composed of calcium-rich plagioclase and pyroxene. Makes up most of the lower oceanic crust (layer 3). Dark, heavy, and commercially sold as 'black granite.'
A high-grade metamorphic rock with characteristic alternating light and dark mineral bands formed by intense heat and pressure. The Acasta Gneiss in Canada, at 4.03 billion years old, is among the oldest known intact rocks on Earth. Pronounced 'nice.'
The backbone of continental crust, composed of quartz, feldspar, and mica. Forms from slowly cooling magma deep underground over thousands to millions of years. The most abundant plutonic rock on Earth's surface. Used extensively in construction, countertops, and monuments.
A dark, hard, poorly sorted sandstone deposited by turbidity currents in deep marine environments. Contains a mix of sand, silt, and clay-sized particles with rock fragments. Common in ancient ocean floor deposits and accretionary wedges at subduction zones.
A fine-grained, hard metamorphic rock produced by contact metamorphism — when existing rock is baked by an adjacent magma intrusion. Lacks the foliation of regionally metamorphosed rocks because pressure wasn't the dominant factor. Dense, tough, and often splintery.
A volcanic rock formed from extremely hot, fast-moving pyroclastic flows — superheated clouds of gas, ash, and rock fragments that travel at up to 700 km/h. The fragments are so hot when deposited that they weld together. Covers vast areas — single ignimbrite sheets can exceed 1,000 km². The Bishop Tuff in California covers 1,500 km².
A sedimentary rock containing at least 15% iron, typically as iron oxides, carbonates, or silicates. Banded iron formations (BIFs) — alternating layers of iron minerals and silica — are 1.8-2.5 billion years old and record the Great Oxidation Event when photosynthetic organisms first oxygenated Earth's atmosphere. Source of most of the world's iron ore.
An opaque variety of chalcedony (microcrystalline quartz) colored by iron oxide inclusions. Found in reds, yellows, browns, and greens. Technically a form of chert, but distinguished by its vivid colors and opacity. Used as a gemstone and ornamental stone since antiquity. Found worldwide in volcanic and sedimentary environments.
A banded metamorphic rock consisting of alternating layers of red jasper and silver-gray hematite (iron oxide). Forms from metamorphism of banded iron formations. Among the most visually striking rocks on Earth. The red and silver striping records ancient ocean chemistry from over 2 billion years ago.
An ultramafic volcanic rock famous as the primary source of natural diamonds. Erupts from depths exceeding 150 km at speeds up to 130 km/h in violent, gas-driven pipe eruptions. Kimberlite pipes are essentially geological cannons that shoot diamonds to the surface.
An extremely rare ultramafic volcanic rock that last erupted naturally over 2 billion years ago, when the Earth's mantle was hot enough (1,600°C+) to produce such magnesium-rich lavas. Named after the Komati River in South Africa. A window into early Earth conditions.
Composed primarily of calcium carbonate (CaCO3), often from marine organism shells and coral. Makes up about 10% of all sedimentary rocks. Dissolves in slightly acidic water, creating caves, sinkholes, and karst topography. The raw material for cement manufacturing.
Metamorphosed limestone composed of recrystallized calcite or dolomite. The interlocking crystal structure gives it a characteristic sugary texture and translucency. Prized since antiquity for sculpture (Michelangelo's David) and architecture (the Taj Mahal, the Parthenon).
A 'mixed rock' that has been partially melted, creating dramatic swirled patterns of light (melted, granitic) and dark (unmelted, metamorphic) bands. Represents the boundary between metamorphism and igneous processes. Found in the cores of ancient mountain belts.
A fine-grained sedimentary rock made of clay and silt-sized particles. Similar to shale but lacks shale's characteristic fissility (tendency to split into thin layers). Forms in quiet water environments — lake beds, deep ocean floors, floodplains.
Formed by intense shearing and grinding along fault zones under ductile conditions. The original rock is stretched, smeared, and recrystallized into thin, flowing bands. Contains porphyroclasts — surviving mineral grains that resisted the deformation around them.
A dense, hard, white-to-gray rock composed of microcrystalline quartz. Found primarily in the Ouachita Mountains of Arkansas and Oklahoma. Used for centuries as whetstones and sharpening stones — the finest natural abrasive material known.
Volcanic glass formed when felsic lava cools so rapidly that crystals cannot form. Fractures with razor-sharp conchoidal edges — obsidian blades can be sharper than surgical steel at the molecular level. Used for tools since the Stone Age and still used in some surgical scalpels.
A limestone composed of ooids — small, rounded grains formed by concentric layers of calcite precipitated in warm, shallow, agitated water (like the Bahamas today). Each ooid is typically 0.25-2mm in diameter. A rock made of tiny geological pearls.
Igneous rock with crystals exceeding 1 cm, sometimes reaching meters in length. Forms from the last, most water-rich stages of magma crystallization. The world's primary source of lithium, beryllium, tantalum, and many rare minerals. Some pegmatite crystals exceed 10 meters.
An olivine-rich ultramafic rock that makes up most of Earth's upper mantle. Dense, green, and rarely seen at the surface except in ophiolite complexes and xenoliths. The rock that diamond-bearing kimberlites punch through. Contains the mineral composition of the planet's interior.
A fine-grained foliated metamorphic rock intermediate between slate and schist. Distinguished by a characteristic silky or pearly sheen on foliation surfaces, caused by microscopic mica crystals just beginning to grow visible. The metamorphic 'teenager' — not quite slate, not yet schist.
A highly vesicular (bubbly) volcanic rock so porous it floats on water. Formed when gas-rich felsic magma is ejected during explosive eruptions and rapidly depressurizes, creating a frozen foam. Used as an abrasive, a lightweight aggregate, and in stone-washed jeans.
Metamorphosed sandstone in which the original quartz grains have been recrystallized into an interlocking mosaic. Extremely hard (Mohs 7) — harder than steel — and resistant to weathering. Fractures through the grains rather than around them, unlike sandstone.
The fine-grained volcanic equivalent of granite. Often light-colored with visible flow banding from viscous lava movement. Associated with explosive volcanic eruptions due to high silica content trapping gas. Yellowstone's supervolcano eruptions produced massive rhyolite deposits.
Composed of cemented sand grains, most commonly quartz. Forms in deserts, rivers, beaches, and shallow marine environments. Layering and cross-bedding preserve records of ancient wind and water currents. Used extensively in construction since ancient Egypt.
A medium-grade foliated metamorphic rock with visible, parallel-aligned mica flakes that give it a characteristic sparkle. Named by its dominant mineral — garnet schist, mica schist, etc. Manhattan's skyscrapers stand on Manhattan Schist, a 450-million-year-old formation.
A dark, vesicular volcanic rock similar to pumice but denser and composed of mafic material. Doesn't float on water (unlike pumice). Common in cinder cones and as the frothy top of lava flows. Used as lightweight aggregate, landscaping material, and barbecue grill lava rocks.
Formed by the hydration and metamorphism of ultramafic rocks like peridotite. Named for its snake-like green patterns. California's state rock. Often found along fault zones and at mid-ocean ridges. Contains asbestos minerals (chrysotile) and can be a health hazard when crushed.
The most common sedimentary rock on Earth, made of compacted clay and silt. Characterized by fissility — the ability to split into thin, flat layers. Preserves fossils beautifully due to fine grain size. Source rock for oil and gas. Target of hydraulic fracturing (fracking).
A sedimentary rock with grain sizes between sandstone and mudstone (0.004-0.0625mm). Feels gritty between your teeth (geologists actually test this). Often forms in quiet river floodplains and lake environments. Less fissile than shale, less gritty than sandstone.
Fine-grained foliated metamorphic rock formed from shale under low-grade metamorphism. Splits perfectly along foliation planes (slaty cleavage) into flat sheets. Used for roofing, flooring, chalkboards, and billiard table surfaces for centuries.
A metamorphic rock composed largely of the mineral talc, making it one of the softest rocks (Mohs 1-2). Heat-resistant and chemically inert. Used for countertops, woodstoves, laboratory benches, carving, and as a marker in metalworking. Soapstone whiskey stones are a modern novelty.
A form of limestone deposited by mineral springs, especially hot springs. Built up in terraced layers as dissolved calcium carbonate precipitates. Creates spectacular natural formations like Mammoth Hot Springs in Yellowstone and Pamukkale in Turkey. Popular as decorative building stone.
Formed from consolidated volcanic ash ejected during explosive eruptions. Can range from soft and crumbly to hard and welded (if deposited hot). The Roman Empire built extensively with tuff, and it's a key ingredient in Roman concrete, which has lasted 2,000+ years.
A sedimentary rock deposited by turbidity currents — underwater avalanches of sediment that race down continental slopes into the deep ocean at speeds up to 80 km/h. Each turbidite bed shows graded bedding: coarse at the bottom, fining upward to clay. Stacked sequences can be thousands of meters thick.
A distinctive green and pink metamorphic rock composed of epidote (green), orthoclase feldspar (pink), and quartz (clear to gray). Forms from the hydrothermal alteration of granite. Named after the Unaka Range of the Appalachian Mountains. Popular as a decorative and lapidary stone.
A finely laminated sedimentary rock formed from varves — annual pairs of light and dark layers deposited in glacial lakes. Each light layer (summer: coarser silt) and dark layer (winter: fine clay settled under ice) represents one year. Counting varves gives precise chronologies stretching back tens of thousands of years.
A limestone classification in the Dunham scheme: more than 10% grains (fossils, shell fragments) supported in a lime mud matrix. Intermediate between mudstone and packstone. Common in deeper shelf environments where gentle currents carried skeletal debris into quieter, muddier settings.
A fragment of pre-existing rock that gets engulfed by magma during volcanic activity and survives without melting completely. The word comes from the Greek xenos (stranger) and lithos (stone). A xenolith is, literally, a stranger stone — a geological immigrant trapped in a country it never applied to enter.
A rare earth phosphate mineral (YPO₄) prized as a source of yttrium and heavy rare earth elements. Found in pegmatites and placer deposits. Named from the Greek for “vain honor” because the discoverer mistakenly thought it contained a new element — it didn’t, but the name stuck. Even the naming was an indignity.
A brilliant white metamorphic marble quarried near the town of Marble, Colorado, at an elevation of 9,300 feet. Formed from Mississippian-age limestone (~330 million years old) recrystallized by contact metamorphism from the Treasure Mountain laccolith. Selected for the Lincoln Memorial and the Tomb of the Unknown Soldier. A 330-million-year-old rock dragged from a mountain at 9,300 feet to become a building in a swamp.
A streamlined ridge of rock carved by persistent wind erosion and sandblasting, found in deserts worldwide and on Mars. Named from the Turkic yar (steep bank). Yardangs can be meters to kilometers long, sculpted from soft sedimentary rock by millions of years of wind carrying sand particles as natural abrasives. The wind does not stop. The rock does not move. The result is sculpture by attrition.
A zirconium silicate (ZrSiO₄) that holds the record for the oldest known material on Earth: a 4.4-billion-year-old crystal from the Jack Hills of Western Australia. Zircon is so chemically stable it preserves uranium-lead ratios that allow precise radiometric dating. It has survived the Hadean bombardment, multiple supercontinent cycles, and 4.4 billion years of planetary upheaval. We use it as a cheap diamond substitute.
A group of over 40 naturally occurring microporous minerals formed when volcanic ash reacts with alkaline groundwater. Their crystal structure contains tiny channels and cavities that can selectively trap molecules by size — nature’s original filter. Used in water purification, cat litter, nuclear waste cleanup, and laundry detergent. From volcanic explosion to absorbing cat urine: the full arc of geological indignity.