Rock and Mineral Identification Key
Hardness alone identifies 2 of 39 minerals. Add specific gravity and it is 21. Colour identifies almost nothing.
No single test identifies a mineral
Across the 39 minerals in this key, here is how many each test pins down to exactly one answer on its own: hardness 2, specific gravity 3, colour 4, lustre 4, streak 7. Not one of them reaches a fifth of the table.
Hardness is the worst of the five, which is awkward, because a scratch test is what almost everyone does first. Streak — rubbing the specimen on unglazed porcelain — is 4 times better and takes five seconds.
The jump is in the pairs. Hardness with specific gravity identifies 21 of 39 outright — 11 times what hardness manages alone, from one extra measurement. And that measurement is not lab work: a kitchen scale, a cup of water and a piece of string will do it.
How much each test is worth
Counted over the 39 minerals in this key, not estimated.
| Test | Answers it can give | Identified outright | Largest group left |
|---|---|---|---|
| Hardness alone | 9 | 2 of 39 | 9 |
| Specific gravity alone | 10 | 3 of 39 | 8 |
| Colour alone | 13 | 4 of 39 | 7 |
| Lustre alone | 9 | 4 of 39 | 21 |
| Streak alone | 12 | 7 of 39 | 23 |
| Specific gravity + streak | 20 | 13 of 39 | 7 |
| Hardness + streak | 24 | 17 of 39 | 4 |
| Hardness + specific gravity | 28 | 21 of 39 | 4 |
Colour is the one to distrust
19 of the 39 minerals here — 49 per cent — occur in three or more colours. Quartz alone comes colourless, white, purple, pink, grey, brown and yellow, and fluorite manages five. Sorting a tray by colour sorts nothing, which is why colour is offered last on this page rather than first.
The streak is different, and this is the useful distinction: it is the colour of the powder, not the specimen, and it stays put when the specimen does not. Hematite can look like steel grey metal or dull red earth and gives a red-brown streak either way. Pyrite looks like gold and streaks greenish black, which is the test that has settled the question for two centuries.
Testing hardness with what you already own
Whatever scratches it is harder; whatever it scratches is softer.
| Object | Roughly Mohs |
|---|---|
| Fingernail | 2.5 |
| Copper coin | 3.5 |
| Steel knife or nail | 5.5 |
| Glass | 5.5 |
| Steel file | 6.5 |
| Quartz / streak plate edge | 7 |
Scratch the unknown with the object, not the other way round, and wipe the mark — a softer object leaves a streak of itself that looks exactly like a scratch. Test a fresh surface and an inconspicuous corner, since a weathered rind is often much softer than the mineral beneath.
How to use
- Start with a streak test — rub it on unglazed porcelain. It beats hardness on its own.
- Add a hardness reading using a fingernail, coin, knife or file.
- Weigh it in air and in water for specific gravity; that is the biggest single gain.
- Leave colour until last. Half these minerals come in three or more colours.
Frequently asked questions
Which test identifies a mineral best?
None of them alone. Counted over the 39 minerals in this key, hardness pins down 2, specific gravity 3, colour 4, lustre 4 and streak 7. Not one reaches a fifth of the table. The useful move is combining two: hardness with specific gravity identifies 21 of 39 outright, which is more than ten times what hardness manages by itself.
Why is hardness the worst single test?
Because the Mohs scale has ten steps and most common minerals crowd into the middle of it. A reading of 6 leaves nine candidates in this key. It is also the easiest test to get wrong — a weathered surface reads much softer than fresh material, and a softer object leaves a mark that looks convincingly like a scratch until you wipe it.
What is a streak test?
Rubbing the specimen on unglazed porcelain — the back of a bathroom tile works — to see the colour of its powder. It takes five seconds and is three times more informative than hardness in this key, because the powder colour is far more consistent than the specimen colour. Minerals harder than about 7 will not leave one; they scratch the plate instead.
Why is colour unreliable?
Because 19 of the 39 minerals here occur in three or more colours. Quartz alone comes colourless, white, purple, pink, grey, brown and yellow; fluorite manages five. Trace impurities change the colour without changing the mineral, so sorting a tray by colour sorts nothing.
How is streak different from colour?
It is the colour of the powder rather than the surface, and it stays put when the surface does not. Hematite can look like steel grey metal or dull red earth and gives a red-brown streak either way. Pyrite looks like gold and streaks greenish black, which is the test that has settled that particular question for two centuries.
How do I test hardness without a kit?
A fingernail is about 2.5, a copper coin about 3.5, a steel knife or nail and ordinary glass about 5.5, and a steel file about 6.5. Scratch the unknown with the object rather than the other way round, wipe the mark to check it is a groove and not a smear, and test a fresh surface rather than a weathered rind.
What is specific gravity and why does it help so much?
How many times heavier the sample is than the same volume of water. It helps because the values are spread widely — from 2.05 for sulphur to 8.1 for cinnabar — while hardness crams everything into ten steps. Two minerals that match on hardness usually differ on weight, which is why the pair is so much better than either alone.
Is measuring specific gravity difficult?
No. Weigh the sample dry, hang it from a thread so it is fully submerged and touching nothing, and weigh it again. Specific gravity is the dry weight divided by the difference. A kitchen scale is enough, and the units cancel so it does not matter whether you use grams or ounces.
What is the Mohs scale exactly?
An ordinal scale defined by ten reference minerals: talc 1, gypsum 2, calcite 3, fluorite 4, apatite 5, orthoclase 6, quartz 7, topaz 8, corundum 9, diamond 10. Ordinal means the steps are not equal — diamond is several times harder than corundum, not ten per cent harder, so a mineral at 8 is not "80 per cent as hard" as one at 10.
Why does the key still show several candidates?
Because a real key narrows rather than decides, and stopping at one answer when the evidence supports three would be dishonest. Every candidate shown is consistent with what you have measured. Adding a test you have not done yet is usually faster than staring at the list — the table on the page says which test buys the most.
Can this identify a rock rather than a mineral?
Not directly. A rock is an aggregate of minerals, so the useful approach is to identify the minerals you can see in it and work back — quartz with feldspar and mica points to granite, olivine and augite to basalt. This key works on individual mineral grains and crystals.
Are the tolerances strict?
Deliberately not. Hardness is matched within half a step and specific gravity within about six per cent, because a scratch test and a kitchen scale are not laboratory instruments. A key demanding exact matches would rule out the right answer more often than the wrong ones.
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