Rocks & minerals
What kind of rock is this?
Rock identification is not about colour. It is about what the rock is made of and how the grains are arranged, and four tests you can do at a kitchen table settle most of it: look at the grain with a hand lens, try to scratch it with a knife, drop vinegar on it, and see whether it has layers. Set what you know and ignore the rest.
Narrow it down
Showing all 33 rocks
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Granite
Coarse interlocking crystals in a salt-and-pepper mix of pink or white feldspar, glassy grey quartz and black flecks of mica or hornblende. No layering, no preferred direction.
Granite: what type of rock it is -
Basalt
Dark grey to black, heavy, fine-grained and often peppered with small round gas holes. Frequently mildly magnetic because of the magnetite in it.
What basalt is and where it forms -
Obsidian
Handle with careJet black, brown or mahogany volcanic glass with a bright glassy shine, breaking into curved shell-like faces with edges sharper than a scalpel.
Obsidian: how volcanic glass forms -
Pumice
Pale grey or cream, so full of gas bubbles that it floats on water — the only common rock that does. Feels like solid foam and abrades skin.
What pumice is -
Scoria
Dark red-brown to black, heavily bubbled, rough and crunchy, sinking in water. The rust colour comes from iron oxidised while the fragment was still hot.
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Rhyolite
The fine-grained equivalent of granite: pale pink, cream or grey, hard, often with flow banding and scattered larger crystals of quartz or feldspar in a fine matrix.
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Andesite
Medium grey to purplish, harder to place than basalt or rhyolite because it sits between them, typically speckled with small rectangular white feldspar crystals.
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Gabbro
Coarse-grained and almost entirely dark: black pyroxene and grey-green plagioclase with essentially no quartz. Distinctly heavier than granite of the same size.
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Diorite
Coarse-grained black and white in roughly equal proportions, with a peppered look and none of granite's pink feldspar or glassy quartz.
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Peridotite
Dense, dark green to olive, made almost entirely of sugary olivine grains. Very heavy in the hand relative to its size.
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Sandstone
Feels like fine sandpaper, and grains rub off on your fingers at a broken edge. Usually tan, buff, red or grey with visible bedding, and often with cross-bedding in the layers.
How sandstone forms -
Limestone
Fizzes vigorously in vinegar or dilute acid, scratches easily with a steel knife, and often contains obvious fossil shells or crinoid discs. Grey, cream or buff.
Limestone: type, formation and uses -
Chalk
Soft white limestone that marks paper, crumbles under a fingernail and fizzes strongly. Made almost entirely of the plates of microscopic algae.
What chalk is made of -
Dolostone
Looks like limestone and behaves differently in acid: no reaction on a solid surface, a slow fizz only after you scratch it to powder. Often weathers to a buff or pinkish colour.
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Shale
Splits into thin flat sheets along the bedding, is soft enough to scratch with a fingernail, and — the field test everyone uses — smells distinctly of wet earth when you breathe on it.
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Siltstone and mudstone
Fine-grained and dull like shale but breaking into blocks rather than sheets. Siltstone feels faintly gritty when rubbed against a tooth; mudstone does not.
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Conglomerate
Rounded pebbles cemented in a finer matrix — a fossilised gravel bank. The rounding is the point: it means the pebbles travelled in water.
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Breccia
Sharp angular fragments locked into a matrix. Angular means the pieces were buried close to where they broke.
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Chert and flint
Very hard, waxy or dull, breaking into curved sharp-edged flakes. Scratches glass easily and cannot be scratched by steel. Grey, black, brown or honey-coloured, often with a white weathered rind.
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Coal
Black, dull to bright and glassy, notably light for its size, leaving black marks on your hands. Bituminous coal is dull and layered; anthracite is hard, bright and does not mark.
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Rock salt
Clear to pink cubic crystals that taste of salt and dissolve in water. The taste test is legitimate here and is what geologists actually do.
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Gypsum rock
So soft a fingernail marks it, white to pink, sometimes as satin-fibrous bands or as clear cleavable plates. Does not fizz, does not taste of salt.
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Travertine and tufa
Banded cream and tan calcium carbonate full of holes, fizzing strongly in acid, sometimes with fossilised plant stems moulded in it. Widely used as floor tile.
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Marble
Recrystallised limestone: fizzes in acid, is soft enough for a knife, and has a sugary sparkle on a broken face because the calcite grains have grown together. Fossils are destroyed by the recrystallisation.
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Quartzite
Looks like sandstone until you break it: the fracture cuts straight through the grains rather than around them, giving a glassy sugary surface. Scratches glass and does not shed grains.
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Slate
Splits into thin flat plates that ring like a tile when tapped, in grey, purple, green or black. Harder than shale and the split direction often cuts across the original bedding at an angle.
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Phyllite
Slate that has gone one grade further: the split surfaces have a silvery silky sheen from mica too fine to resolve, and they are crinkled rather than flat.
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Schist
Obviously sparkly, splitting along wavy surfaces paved with aligned mica flakes, often studded with garnet or staurolite crystals that grew during metamorphism.
Identifying metamorphic rocks -
Gneiss
Coarse-grained with the dark and light minerals segregated into alternating bands, sometimes swirled or folded. Unlike schist it does not split easily along the banding.
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Amphibolite
Dark, heavy and dominated by black needle-like hornblende crystals, often with white plagioclase between them and a weak alignment rather than true banding.
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Soapstone
Grey-green, and unmistakable by touch: it feels like dry soap and a fingernail carves it. Carvers and woodstove makers use it because it is soft and holds heat.
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Serpentinite
Handle with careMottled dark and light green with a waxy or greasy sheen and polished slippery surfaces, frequently veined with white. California's state rock.
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Hornfels
A dense, hard, dark, featureless rock with a splintery fracture, formed where an intrusion baked the surrounding mudstone. Found as an aureole around a granite body.
Nothing in this key matches every filter at once. That usually means one observation is off — a colour read in shade, or a size guessed rather than measured. Clear the filter you are least sure about and look again.
Four tests, in the order that actually helps
The classic school method — decide igneous, sedimentary or metamorphic first — is the wrong way round for a beginner, because that is the conclusion rather than the observation. Work from what you can measure.
- Break it, then look at the fresh face with magnification. A weathered surface tells you about the weather. Everything in this key refers to a fresh break. A 10× hand lens costs almost nothing; a phone camera at maximum zoom is a workable substitute.
- Scratch test. Fingernail is about 2.5 on the Mohs scale, a copper coin about 3.5, a steel knife or a nail about 5.5, and window glass about 5.5 too. Scratch a fresh face, then rub the powder away with a thumb: soft minerals leave a groove, hard ones leave only a smear of metal.
- Acid test. Put a drop of white vinegar on a fresh face. A vigorous fizz means calcite, so limestone, chalk, marble or travertine. A weak fizz only after you have scratched it to powder means dolomite. No fizz rules out the whole carbonate group in one second.
- Heft and magnetism. Pick up two rocks of similar size and compare. Anything conspicuously heavy is iron-rich or metal-bearing. A fridge magnet on a string picks up magnetite-bearing basalt, magnetite ore and most meteorites.
Those four take under a minute together and eliminate most of the catalogue. Colour, which is what everybody reaches for first, eliminates almost nothing: granite is pink, white, grey or black, and limestone is any colour a trace of iron or organic matter chooses to make it.
The three families, told apart by texture
Once you have the texture, the family follows almost automatically.
| What you see | What it means | Family |
|---|---|---|
| Interlocking crystals, no gaps, no preferred direction | Crystallised from a melt at depth | Igneous, intrusive |
| Fine or glassy, often with gas holes | Cooled fast at or near the surface | Igneous, volcanic |
| Separate grains stuck together, flat layers | Deposited by water, wind or ice, then cemented | Sedimentary |
| Fossils present | Formed at surface temperatures | Sedimentary, essentially always |
| Minerals aligned into bands or splitting planes | Recrystallised under directed pressure | Metamorphic, foliated |
| Recrystallised but uniform, no direction | Heated without directed stress, or made of one mineral | Metamorphic, non-foliated — marble, quartzite, hornfels |
Two traps live in this table. Fossils are decisive evidence for a sedimentary origin, but their absence proves nothing — most sedimentary rock has none. And "no visible grains" is ambiguous on its own: basalt, shale, chert and hornfels are all fine-grained and belong to three different families. That is why the hardness and acid tests matter, and why the pairs section below is worth reading.
The five pairs everybody gets wrong
Granite and gneiss
Same minerals, same coarse grain, same hardness. Granite's crystals are randomly oriented; gneiss has them segregated into light and dark bands, often folded. Look at the whole specimen rather than one face.
Sandstone and quartzite
Break both. Sandstone fails around the grains and leaves a gritty surface that sheds sand; quartzite fails through the grains and leaves a glassy fracture. Rubbing a broken edge on your palm settles it in a second.
Limestone and dolostone
A vinegar drop on a solid face: limestone fizzes visibly, dolostone does almost nothing until you powder it with a knife. Dolostone also tends to weather buff-brown while limestone weathers grey.
Marble and quartzite
Both are pale, massive and sugary. Marble is soft — a steel knife bites into it — and fizzes. Quartzite scratches glass and does nothing in acid.
Shale and slate
Shale is soft enough to scratch with a fingernail, breathes out an earthy smell when damp and splits along its original bedding. Slate is hard, rings when tapped, and its splitting plane frequently cuts across the bedding at an angle — which is exactly why it makes roofing.
Where the rock came from is evidence too
A rock found loose in a field has usually travelled, and one found in place has not. That distinction is the difference between a guess and an identification.
Bedrock exposures — a road cut, a quarry face, a stream bank, a cliff — show the rock where it formed, along with its neighbours and its structures. Identifying from an outcrop is far easier than from a pebble, because you can see whether it is layered, how thick the beds are, and what sits above and below.
Glacial erratics across the northern US, Canada and northern Europe were carried hundreds of kilometres by ice, which is why a lump of Canadian Shield gneiss turns up in an Illinois cornfield. If you are north of the last glacial limit, a stray boulder tells you nothing about the local geology.
River gravels mix everything upstream into one bar, rounded and polished so the original textures are hard to read. Break a river pebble before you try to identify it.
Fill, ballast and landscaping stone is the single most common source of confusing specimens near houses. Railway ballast is usually a hard igneous or metamorphic rock brought from a distant quarry, and bagged landscaping "lava rock" is scoria that may have come from another continent.
Checking your local geology takes a minute and dramatically narrows the possibilities. In the United States the Macrostrat map gives the bedrock unit under any point; most national and state geological surveys publish an equivalent.
A note on what a rock is not
Several things that look like rocks are not, and they account for a large share of unresolvable identifications.
Slag from historic smelting is everywhere in old industrial regions and in fields ploughed near old forges. It is glassy, bubbly, often blue, green or iridescent, sometimes very heavy, and it is the leading candidate whenever someone believes they have found a meteorite.
Concrete and asphalt fragments weather into convincing conglomerates and breccias. Look for an unnaturally uniform matrix and for crushed rock fragments with saw marks.
Meteorites are genuinely rare and the overwhelming majority of suspected finds are slag, magnetite, hematite or ironstone concretions. The useful screening characters: a fresh fusion crust, no vesicles, no quartz, a density well above ordinary rock, and — for most types — a magnet response. Then check for the Widmanstätten pattern on a cut and etched face, which cannot be faked by any terrestrial process.
Concretions — cemented lumps that grew inside sediment — are the source of most "fossil egg" and "dinosaur bone" reports. They are usually spherical or lens-shaped with concentric internal layers and no bone texture.
What this key covers, and what to do next
33 rock types, chosen as the ones people actually pick up: the common igneous suite, the everyday sedimentary rocks, the standard metamorphic series, and the building and landscaping stones that get confused with all of them. That is close to complete for casual identification in North America and Europe and nowhere near complete as geology — there are hundreds of named rock types, and dozens of ore, evaporite and volcanic varieties are absent.
If you get to two candidates and stop, the useful next step is a streak plate and a hand lens rather than more looking. Once the rock is identified, the minerals in it are the more interesting question, and that needs a different key.
For anything that might be valuable or unusual, a local rock and mineral club or a university geology department will look at a specimen for free, and they will be far more use than any photograph. Bring a fresh broken face, not a tumbled or weathered one.
Questions people ask about this
How do I identify a rock without any equipment?
Four things are enough: break it and look at the fresh face closely, try to scratch it with a fingernail and then a steel knife, drop white vinegar on it and watch for fizzing, and compare its weight against another rock the same size. That sequence sorts most common rocks into a small group. Colour is the least useful character and the one most people start with.
What rock fizzes in vinegar?
Anything made of calcite: limestone, chalk, marble and travertine all fizz vigorously on a solid face. Dolostone and dolomitic marble fizz only feebly, and only after you scratch the surface to powder. Nothing else in this key reacts, which makes one drop of vinegar the fastest single test in rock identification.
Is this rock a meteorite?
Almost certainly not — the overwhelming majority of suspected meteorites are smelting slag, magnetite, hematite or ironstone concretions. Screening characters that rule one out quickly: gas bubbles or vesicles, visible quartz, layering, a streak on unglazed porcelain, or ordinary density. Genuine meteorites have a dark fusion crust, are notably dense, usually attract a magnet, and iron ones show a Widmanstätten pattern when a cut face is polished and etched.
What is the difference between granite and gneiss?
The minerals are the same and the arrangement is not. Granite crystallised from a melt, so its quartz, feldspar and mica are randomly oriented. Gneiss was squeezed and recrystallised, so the light and dark minerals are segregated into alternating bands, often folded. If it is stripey, it is gneiss.
How can I tell sandstone from quartzite?
Break both and look at the fracture. Sandstone breaks around the sand grains, leaving a gritty surface that sheds sand when rubbed. Quartzite breaks straight through the grains, leaving a glassy, sugary surface that sheds nothing. Quartzite also scratches glass easily, while poorly cemented sandstone will not.
Why does my rock have holes in it?
If they are rounded and the rock is dark and heavy, they are gas bubbles frozen into a lava flow, which makes it vesicular basalt or scoria. If the rock is pale and light enough to float, it is pumice. If the holes are irregular and the rock fizzes in acid, it is travertine or a weathered limestone where acid groundwater has dissolved parts away.
Can I identify a rock from a photograph?
Partly. A photograph of a fresh broken face at close range, with something for scale and light coming from the side, gets you the texture and often the family. What a photograph cannot give is hardness, heft or the acid reaction, and those are the tests that separate the pairs people actually confuse. Do the three physical tests first, then photograph.
Still not sure? Point a camera at it
A filter narrows rocks down to a shortlist. Orvik reads the photo itself and names the species, with the look-alikes ranked underneath so you can check the call yourself.