Most beginners reach for colour first, but colour is one of the least reliable clues in mineralogy. Quartz alone appears colourless, purple, pink, grey and brown depending on trace impurities. A more dependable approach is to test a small set of physical properties in sequence and let them narrow the field. None of the steps below require more than a few inexpensive tools.
Hardness
Hardness measures a mineral's resistance to being scratched. The reference scale was devised by the mineralogist Friedrich Mohs in 1812 and runs from talc, the softest, to diamond, the hardest. It is a relative scale: a mineral can scratch anything below it and is scratched by anything above it.
| Mohs value | Reference mineral | Common test object |
|---|---|---|
| 1 | Talc | — |
| 2 | Gypsum | Fingernail (about 2.5) |
| 3 | Calcite | Copper coin (about 3) |
| 5–5.5 | Apatite | Steel knife or nail |
| 6.5 | — | Streak plate / glass (about 5.5–6.5) |
| 7 | Quartz | Scratches glass |
A practical first test: try to scratch the specimen with a fingernail, then a copper coin, then a steel blade, then ordinary glass. If the mineral scratches glass it is at least about 6 on the scale, which already rules out a large number of soft species.
Streak
Streak is the colour of a mineral's powder, seen by dragging it across an unglazed porcelain tile known as a streak plate. The powder colour is frequently more consistent than the colour of the solid piece. Pyrite, for example, looks brassy gold but leaves a greenish-black streak — a quick way to separate it from actual gold, which leaves a yellow streak.
Lustre
Lustre describes how a surface reflects light. The broad division is between metallic and non-metallic. Within non-metallic lustre, useful descriptive terms include vitreous (glass-like, as in quartz), pearly, silky, resinous and earthy. Lustre is subjective, so it is best used alongside the harder evidence of hardness and streak rather than on its own.
Cleavage and fracture
When a mineral breaks, it either splits along flat planes (cleavage) or breaks along irregular surfaces (fracture). Cleavage reflects the internal atomic structure and is highly diagnostic. Mica peels into thin elastic sheets because it has one perfect cleavage direction. Fluorite breaks along four directions, producing octahedral fragments. Quartz, by contrast, has no cleavage and instead breaks with the curved, shell-like surface called conchoidal fracture.
Worked example: telling quartz from calcite
Both can appear as clear or pale crystals, so beginners confuse them. Two quick tests separate them cleanly:
- Hardness: quartz scratches glass; calcite does not, and is itself scratched by a steel blade.
- Cleavage: calcite breaks into neat rhombic fragments along three cleavage directions; quartz fractures conchoidally with no flat planes.
Field note
One classic confirmatory test for calcite is its reaction to dilute acid: a drop of weak hydrochloric acid will fizz on calcite. Use such chemical tests sparingly, follow safety guidance, and never apply them to a valuable or borrowed specimen.
Habit and other clues
Crystal habit is the characteristic outward shape a mineral tends to take — cubic, prismatic, bladed, fibrous or massive. Combined with the tests above, habit often closes the case. Additional properties such as magnetism (magnetite), a distinctive feel, or unusually high density can provide further confirmation for specific minerals.
Worked together, these observations let a beginner reach a confident provisional identification by hand. For uncertain or scientifically important specimens, identification should be confirmed by laboratory methods rather than physical tests alone.
For deeper reference data on individual species, the community database Mindat.org catalogues localities and properties, and the U.S. Geological Survey provides general background on minerals at usgs.gov.