Putting a name to a stone takes a little method. Geologists don't identify rocks by colour alone; they run through a short series of observations and simple tests, each of which rules some candidates in and others out. This guide follows that same order, using things you already have at home.
Rock or mineral?
It helps to know which you're holding, because the tests mean different things for each.
- A mineral is a naturally occurring solid with a specific chemical composition and an ordered internal structure. Quartz, calcite, feldspar and pyrite are minerals. A single mineral tends to look the same all the way through.
- A rock is a mixture of one or more minerals, sometimes with glass or fragments of other rocks. Granite, for example, is mostly quartz, feldspar and mica. Look closely and you'll often see several different grains.
If your stone shows distinct grains of different colours, it's almost certainly a rock. If it's one uniform material, perhaps with flat crystal faces, you may have a single mineral. Most of the tests below (streak, hardness, luster) work best on a single mineral or on one mineral grain within a rock.
1. Clean it and find a fresh surface
Weathering hides a stone's real character. Iron staining, lichen, dust and a dull outer rind can make a pale rock look brown or a shiny one look dull.
- Wash it with water and a soft brush. Avoid acids and bleach, which can damage some minerals.
- Look for a broken or freshly exposed surface.
- Use a hand lens or a magnifier and good daylight. A 10x loupe is the single most useful tool in rock identification.
2. Look at colour, grain size and texture
Colour is the first thing everyone notices and the least reliable on its own. Quartz alone can be clear, white, purple, pink, smoky brown or yellow, depending on tiny impurities. Use colour as a clue, not a verdict.
Grain size and texture tell you much more:
- Coarse grains you can see with the naked eye suggest a rock that cooled slowly underground, like granite, or a sedimentary rock made of sand or pebbles.
- Fine grains that need a lens suggest fast cooling, like basalt, or fine sediment, like shale.
- Glassy with no grains at all suggests volcanic glass such as obsidian.
- Holes and bubbles (vesicles) point to lava, as in pumice and scoria.
- Layers suggest sediment deposited over time.
- Bands or a sheen of aligned flaky minerals suggest a metamorphic rock.
3. Check the luster
Luster is how a surface reflects light. Look at a fresh, clean face and ask which word fits best:
- Metallic: shines like polished metal (pyrite, galena).
- Vitreous (glassy): like broken glass (quartz).
- Pearly: soft sheen like the inside of a shell (some micas, talc).
- Silky: fibrous shimmer (satin spar gypsum).
- Greasy or waxy: looks slightly oiled (some quartz varieties, nepheline).
- Earthy or dull: no shine at all (clay, chalk).
The first split, metallic versus non-metallic, is one of the most useful in any identification key.
4. Do a streak test
The streak is the colour of a mineral's powder. Rub a corner of the mineral firmly across the back of an unglazed white tile (the rough, unglazed underside of a bathroom tile works). The powder left behind is often more consistent than the colour of the specimen itself.
| Mineral | Typical colour | Streak |
|---|---|---|
| Hematite | Black, silver-grey or red | Red-brown |
| Magnetite | Black | Black |
| Pyrite | Brassy yellow | Greenish-black |
| Gold | Golden yellow | Golden yellow |
| Goethite / limonite | Brown, yellow-brown | Yellow-brown |
| Quartz, feldspar | Various | None (harder than the tile) |
An unglazed tile has a hardness of about 6.5 to 7. Minerals harder than that will scratch the tile instead of leaving a streak, which is itself a useful clue.
5. Test the hardness
Hardness is resistance to scratching, measured on the Mohs scale from 1 (talc) to 10 (diamond). You can estimate it with a fingernail, a copper coin, a steel knife and a piece of glass. Try to scratch a fresh, inconspicuous spot on the stone, then rub the mark with your finger to check it's a real groove and not a smear of metal or powder.
Hardness is one of the best ways to separate look-alikes: quartz (7) easily scratches glass, while calcite (3) can't. Our guide to the Mohs hardness scale at home covers the method in detail and how to avoid the common mistakes.
6. Try a magnet
Hold a strong magnet (a fridge magnet is often too weak; a small neodymium magnet is ideal) near the stone.
- Strongly attracted: likely magnetite, or a rock rich in it. Iron-nickel metal, as in many meteorites, also responds strongly.
- Weakly attracted: may contain some magnetite or pyrrhotite. Many dark volcanic rocks, such as basalt, pull gently on a magnet.
- No attraction: the large majority of rocks and minerals.
A magnetic stone isn't necessarily special, but if it's also heavy and has a dark crust, read how to tell if a rock is a meteorite.
7. Feel the weight
Density (or specific gravity) is the weight of a material compared to the same volume of water. Most common rocks, including granite, sandstone and limestone, sit around 2.5 to 3 times the weight of water. With practice you can feel the difference in your hand.
Compare your stone with a similar-sized ordinary rock. If it feels noticeably heavy for its size, it may contain metal or dense minerals such as barite, galena, hematite or magnetite. If it feels unusually light, it may be pumice, a porous rock, or not a rock at all (glass, slag or plastic can turn up in a pile of stones).
8. Look for cleavage and fracture
How a mineral breaks is built into its atomic structure:
- Cleavage means it splits along flat, shiny planes. Mica peels into thin sheets; calcite breaks into slanted blocks (rhombs); halite breaks into cubes.
- Fracture means it breaks irregularly. Quartz and glass show conchoidal fracture, smooth curved surfaces like the inside of a shell.
If you're trying to decide between clear stones, see quartz, calcite or glass?
Work out the rock type
Once you've made your observations, place the rock in one of the three main families. This alone narrows the possibilities a great deal.
| Rock type | How it forms | Clues | Examples |
|---|---|---|---|
| Igneous | Cools from molten magma or lava | Interlocking crystals, no layers; glassy or bubbly if volcanic | Granite, basalt, obsidian, pumice |
| Sedimentary | Layers of sediment compacted and cemented | Visible grains or pebbles, layering, sometimes fossils; often soft | Sandstone, shale, limestone, conglomerate |
| Metamorphic | Existing rock changed by heat and pressure | Banding, aligned flaky minerals, sparkly sheen; very compact | Slate, schist, gneiss, marble, quartzite |
Some quick rules of thumb: a rock with fossils is almost always sedimentary, and a rock that fizzes in acid is probably limestone or marble (both made of calcite). Location helps too: free geological maps from national surveys show which rocks occur where you found it.
Confirm your answer
When your tests point to a few candidates, check them against a good field guide, a museum collection, or an online mineral database. Compare all your observations, not just the photo: a match in colour but a mismatch in hardness or streak means you need to keep looking.
A photo identification can speed this up. AI Stone gives a likely name along with the expected streak, hardness, luster, magnetism and density for that rock or mineral, so you can confirm the suggestion with your own streak and scratch tests. A photo can't measure hardness or chemistry, though, so the hands-on tests are what settle close calls.
Tip: write down each observation as you go, in the same order every time: colour, luster, streak, hardness, magnetism, weight, breakage. A short note makes it far easier to compare your stone against references, or to ask a local rock club for help.
If your find looks unusual, valuable or possibly from space, take it to a local mineral club, a university geology department or a natural history museum. Experts can run tests that no home kit or photo can, and many are happy to help.