The five field tests
Mineral identification is not pattern-matching against photographs. It is a short sequence of physical tests, run in a particular order, each of which eliminates a large fraction of the possibilities. The whole kit costs less than a takeaway meal, and four of the five items are probably already in your house.
Run them in the order below. Hardness and streak first, because they are the most reliable; colour last, because it is the least.
1
Hardness
- What it tells you
- Resistance to scratching, ranked on the Mohs scale from 1 to 10. It is the single most discriminating property available without a laboratory, because it varies widely between minerals and barely at all within one.
- How to run it
- Break a fresh surface. Press firmly and drag once. Wipe the spot with your thumb, then look — a metal smear wipes away, a real scratch does not.
- What goes wrong
- Testing a weathered crust, or testing a porous rock like sandstone where you are really measuring the cement between grains.
- Kit
- Fingernail, copper wire, steel knife, glass, a quartz point
2
Streak
- What it tells you
- The colour of the mineral in powdered form. It strips away the surface effects that make bulk colour unreliable, and for the metallic minerals it is close to decisive.
- How to run it
- Drag the specimen firmly across the plate and look at the powder left behind. Blow away any loose material first.
- What goes wrong
- Minerals harder than about 6.5 scratch the plate instead of powdering. The white line you see is plate, not mineral.
- Kit
- The unglazed back of a ceramic tile
3
Lustre
- What it tells you
- How the surface reflects light. The first and most useful split in all of mineralogy is metallic versus non-metallic, and it divides the candidate list roughly in half before you have done anything else.
- How to run it
- Look at a fresh surface in daylight, turning it slowly. Ask one question first: does this look like metal? If yes, expect a dark streak and high density.
- What goes wrong
- Tarnish. A tarnished metallic mineral can look dull and earthy. Break a corner off and judge the fresh face.
- Kit
- Good light and a freshly broken face
4
Cleavage and fracture
- What it tells you
- How the mineral breaks. Cleavage is breakage along flat planes determined by the crystal structure; fracture is everything else. The number of cleavage directions and the angles between them are strongly diagnostic.
- How to run it
- Hold a broken piece under a lamp and rotate it slowly. Cleavage faces flash as whole planes, all at once. Fracture surfaces produce a travelling glint.
- What goes wrong
- Confusing crystal faces with cleavage faces. Crystal faces grew that way; cleavage faces were made when it broke. Look for repeated parallel steps, which indicate cleavage.
- Kit
- A bright lamp
5
The special tests
- What it tells you
- Individually narrow tests that are close to conclusive when positive. A magnet sticking to a rock takes you from forty-seven candidates to one.
- How to run it
- Magnet for magnetite. Vinegar for carbonates — fizzing means calcite, fizzing only on scratched powder means dolomite. Heft in the hand for the heavy minerals such as barite and galena. Touch for the greasy feel of talc and graphite.
- What goes wrong
- Never taste an unknown mineral. The salt test applies only once you have already narrowed to halite, and lead, mercury and arsenic minerals are all things people have licked.
- Kit
- A magnet, white vinegar, your hands, a UV lamp if you have one
Now narrow it down
Feed your results in and watch the list collapse. You do not need every answer — three good ones usually get you to a handful of candidates.
Start identifying