The kit you need
You do not need a laboratory. With a few euros you can put together a field kit that solves most identifications:
- a 10x hand lens to look at small crystals and surfaces;
- an unglazed porcelain tile (the back of a bathroom tile works) to test the streak;
- a steel nail or pocket knife, a piece of glass and a copper coin to estimate hardness;
- a small magnet and, if you want, a few drops of vinegar.
1. Colour: useful, but often misleading
Colour is the first thing we notice and the least reliable. Many minerals take their colour from tiny impurities: quartz can be colorless, white, pink, purple (amethyst), yellow (citrine) or smoky brown. On the other hand, some minerals always have the same colour, such as the green of malachite or the blue of azurite. Use colour as a clue, never as the answer.
2. Streak: the true colour of the powder
Rub the mineral on the unglazed porcelain and look at the colour of the line it leaves. The streak is much more constant than the colour of the specimen. The classic example is hematite: it can look black or silvery, but its streak is always reddish brown. Pyrite, golden on the outside, leaves a greenish black streak. Minerals harder than the tile (about 7 on the Mohs scale) leave no streak: they scratch the porcelain instead.
3. Luster: how the surface reflects light
Luster describes how light bounces off a fresh surface. The first distinction is between metallic (like a piece of metal: pyrite, galena) and non-metallic. Non-metallic lusters have their own names:
- vitreous, like glass (quartz, most gems);
- pearly, like the inside of a shell (talc, some micas);
- silky, made of fine fibers (satin spar gypsum);
- greasy or resinous (nepheline, sphalerite);
- adamantine, extremely brilliant (diamond, zircon);
- earthy or dull, without reflections (kaolinite, limonite).
4. Hardness: what scratches what
Hardness is the resistance of a mineral to scratching, measured on the Mohs scale from 1 (talc) to 10 (diamond). In the field you compare your sample with common objects: a fingernail is about 2.5, a copper coin about 3.5, glass and a steel knife about 5.5. If your mineral scratches glass, it is harder than 5.5; if your fingernail scratches it, it is softer than 2.5. We explain the method step by step in our guide to the Mohs hardness scale.
5. Cleavage and fracture: how it breaks
When a mineral breaks along flat, shiny planes it has cleavage, a direct consequence of its atomic structure. Micas split into thin sheets (one direction), halite breaks into small cubes (three directions at 90°), calcite into rhombohedra (three directions not at 90°). Minerals without cleavage show a fracture: quartz and obsidian have a conchoidal fracture, curved like the inside of a shell.
6. Density: feel the weight
Take two pieces of the same size in your hands: the difference in weight is often surprising. Quartz has a specific gravity of about 2.65, barite about 4.5 and galena about 7.5. A sample that feels "too heavy for its size" points to metal-bearing minerals such as galena, barite or magnetite.
7. Crystal shape and habit
When crystals are well formed, their geometry is a powerful clue. Pyrite, fluorite and halite often form cubes; quartz grows in six-sided prisms with a pointed tip; garnet forms rounded dodecahedra; tourmaline makes long prisms with a rounded triangular section. Habit also counts: needles, fibers, plates, botryoidal masses like a bunch of grapes (malachite, hematite).
Special tests
- Magnetism: magnetite is attracted by an ordinary magnet.
- Reaction to acid: calcite fizzes with dilute hydrochloric acid; at home a few drops of vinegar give a weaker but visible reaction on powdered calcite.
- Fluorescence: some fluorites and calcites glow under ultraviolet light.
Never taste or lick unknown minerals: some contain toxic elements such as arsenic, lead or mercury. Wash your hands after handling specimens.
Putting it together: gold or "fool's gold"?
A single test is rarely enough, but three or four together usually are. This is how you tell apart three golden minerals that are often confused:
| Test | Gold | Pyrite | Chalcopyrite |
|---|---|---|---|
| Streak | golden yellow | greenish black | greenish black |
| Hardness (Mohs) | 2.5–3 | 6–6.5 | 3.5–4 |
| Specific gravity | about 19 | about 5 | about 4.2 |
| Behavior | malleable, bends | brittle, often in cubes | brittle, often iridescent tarnish |
How GeoCrystal helps
In GeoCrystal you can photograph a sample and get a list of the most likely minerals, each with its probability and a link to the datasheet. The datasheet lists exactly the properties described here (streak, luster, hardness, cleavage, density, crystal system), so you can confirm the suggestion with your own tests. The app also includes a practical Mohs hardness test guide and lets you catalogue your specimens with photos, place and notes.
GeoCrystal · Mineralogy
WildLife · Nature & hiking