Rock and Mineral Identification Key
Identify a mineral from properties you can test at home: hardness, streak, lustre, cleavage, specific gravity, magnetism, and acid reaction. Lists every matching mineral and what to test next.
Rock and Mineral Identification Key
Enter the physical properties you can test at home and this key lists every mineral consistent with them, and tells you which test would narrow the list fastest.
Reference properties
| Mineral | Hardness | Streak | Specific gravity | How often found |
|---|---|---|---|---|
| Quartz | 7 | White | 2.65 | Common |
| Calcite | 3 | White | 2.71 | Common |
| Dolomite | 3.5–4 | White | 2.85 | Common |
| Pyrite | 6–6.5 | Greenish black, Brownish black | 4.9–5.2 | Common |
| Chalcopyrite | 3.5–4 | Greenish black | 4.1–4.3 | Common |
| Magnetite | 5.5–6.5 | Black | 5.1–5.2 | Common |
| Hematite | 5–6 | Red-brown | 5.2–5.3 | Common |
| Galena | 2.5–2.75 | Lead grey | 7.4–7.6 | Common |
| Fluorite | 4 | White | 3–3.3 | Common |
| Gypsum | 2 | White | 2.3–2.4 | Common |
| Halite | 2.5 | White | 2.1–2.2 | Common |
| Talc | 1 | White | 2.7–2.8 | Common |
| Graphite | 1–2 | Black | 2.1–2.3 | Common |
| Orthoclase feldspar | 6 | White | 2.55–2.63 | Common |
| Plagioclase feldspar | 6–6.5 | White | 2.6–2.76 | Common |
| Muscovite mica | 2–2.5 | White | 2.7–3 | Common |
| Biotite mica | 2.5–3 | White | 2.8–3.4 | Common |
| Hornblende | 5–6 | White, Grey | 3–3.4 | Common |
| Augite | 5.5–6 | White, Grey | 3.2–3.6 | Common |
| Olivine | 6.5–7 | White | 3.2–4.4 | Common |
| Garnet | 6.5–7.5 | White | 3.5–4.3 | Common |
| Apatite | 5 | White | 3.1–3.2 | Less common |
| Topaz | 8 | White | 3.4–3.6 | Less common |
| Corundum | 9 | White | 3.9–4.1 | Less common |
| Barite | 3–3.5 | White | 4.3–4.6 | Common |
| Malachite | 3.5–4 | Light green | 3.6–4 | Less common |
| Azurite | 3.5–4 | Light blue | 3.7–3.9 | Less common |
| Sulfur | 1.5–2.5 | White, Yellow | 2–2.1 | Less common |
| Serpentine | 3–5 | White | 2.5–2.6 | Common |
Documentation
This tool is an identification key for rocks and minerals. Enter the physical properties you can test at home and it lists every mineral in its reference set that fits, explains why the others were ruled out, and suggests which test would narrow the list fastest.
Why a photograph is not enough
The United States Geological Survey answers this plainly: rocks and minerals must be examined in person from all perspectives, because they are extremely difficult to identify from photographs.
The reason is that the properties which identify a mineral are mostly not visual. Streak, hardness, cleavage, density and the reaction to acid all require handling the specimen. None of them appear in an image.
Colour, the one property a photo shows well, is the least reliable of all. Quartz alone occurs colourless, white, pink, purple, grey, brown and black. Two minerals of the same colour can be entirely different, and one mineral can appear in a dozen colours. That is why this key weights colour lowest and streak highest.
The tests
Hardness
Hardness is measured on the Mohs scale from 1 to 10, defined by ten index minerals: talc 1, gypsum 2, calcite 3, fluorite 4, apatite 5, orthoclase 6, quartz 7, topaz 8, corundum 9, diamond 10.
It is tested by scratching. A fingernail is about 2.5, a copper coin about 3.5, a steel knife or nail about 5.5, and window glass about 5.5. If the specimen scratches glass but not a steel file, it is roughly 6 to 6.5.
Streak
Streak is the colour of the powder a mineral leaves when rubbed on unglazed porcelain. It is far more consistent than the colour of the specimen itself. Hematite is the classic case: the mineral can look silvery, black or red, but the streak is always red-brown.
Streak only works for minerals softer than the porcelain plate, about 7.
Lustre
Lustre is how the surface reflects light. The main split is metallic against non-metallic. Non-metallic lustres are described as glassy, pearly, silky, greasy, resinous, dull or brilliant.
Cleavage
Cleavage is the tendency to break along flat planes, and the number of directions is diagnostic. Mica splits into sheets in one direction. Feldspar has two directions. Calcite has three, giving rhombs. Fluorite has four, giving octahedra. A mineral with no cleavage, such as quartz, breaks with curved fracture surfaces instead.
Specific gravity
Specific gravity compares a mineral's density with water. Most common minerals sit between 2.5 and 3. Anything noticeably heavier is a strong clue: barite is about 4.5, which is startling for a pale mineral, and galena is about 7.5.
It can be measured by weighing the specimen dry, then weighing it suspended in water, and dividing the dry weight by the weight lost.
Magnetism and acid
Magnetite is strongly magnetic, which almost identifies it on its own. A drop of dilute hydrochloric acid makes calcite fizz readily; dolomite fizzes only when scratched to powder first. These two tests are quick and decisive.
How the key works
Each property you enter is compared against reference values. A mineral stays on the list only if nothing you observed contradicts it. Untested properties never count against a mineral, so a partial set of tests still gives a useful answer.
The tool never reports a confidence percentage, because there is no honest way to compute one from a handful of hand tests. It reports which minerals remain possible.
Example
A specimen is dark and metallic-looking. Colour alone leaves magnetite, hematite, galena, graphite and several others open.
Rubbing it on porcelain gives a red-brown streak. That single test rules out every candidate except hematite, whatever the specimen looks like.
Had the streak been black and the specimen strongly magnetic, the answer would have been magnetite instead.
Frequently asked questions
Can an app identify my rock from a photo?
Not reliably. No published study measures the accuracy of general image models on ordinary mineral photographs, and the accuracy figures quoted by consumer apps come from their own marketing. Research models that do work well feed in hardness as a separate input, precisely because the image alone is not enough.
Why does the tool sometimes leave several minerals on the list?
Because the tests entered genuinely do not separate them. Hardness 7 with no cleavage and a white streak fits both quartz and garnet; their densities differ, so measuring specific gravity settles it.
My rock does not match anything. Why?
Most rocks are mixtures of several minerals rather than a single mineral. Granite, for example, contains quartz, feldspar and mica together. Test an individual grain rather than the whole rock.
Is a valuation possible from these properties?
No. Worth depends on quality, size, crystal form and locality, none of which follow from a property list. Anything that might be valuable should be seen by a professional appraiser or a university geology department.
How many minerals does this cover?
The reference set holds the common rock-forming and collectable minerals a beginner is likely to meet. Rarer species need a full mineralogical reference and, often, laboratory analysis.