How to Identify Minerals: Hardness, Cleavage, Luster

Working mineral identification needs almost nothing: your fingernail, a copper coin, a steel knife blade or nail, a piece of glass, and a small unglazed porcelain plate (a streak plate). With those and an understanding of five properties, you can identify the great majority of common minerals. The trick is to run the tests in order, letting each result narrow the choices.

Hardness is resistance to scratching, ranked on Mohs scale from 1 (talc, scratched by a fingernail) to 10 (diamond, scratched by nothing natural). The test is simple: try to scratch the unknown with each test object (and vice-versa). If your fingernail (~2.5) scratches it, its hardness is under 2.5; if it scratches glass (~5.5) it is harder than 5.5. Two or three tries bracket the answer.

Hit a mineral and it breaks. If it breaks along flat, shiny planes, that is cleavage — the mineral splitting along planes of weak bonds in its lattice. If it breaks irregularly, with curved (conchoidal) surfaces like broken glass, that is fracture.

Cleavage is a powerful clue because the number and angle of cleavage directions reflect the crystal structure:

• Mica — one perfect direction; peels in thin sheets.
• Feldspar — two directions at about 90°.
• Halite — three directions at 90°; cleaves into cubes.
• Calcite — three directions not at 90°; cleaves into rhombohedra.
• Quartz — no cleavage; conchoidal fracture (its framework has no weak plane).

Luster is the way a mineral's surface reflects light. The first split is the most useful: metallic (looks like metal — pyrite, galena, gold, magnetite) versus non-metallic (glassy/vitreous like quartz, pearly like mica, earthy like kaolinite). A metallic luster immediately points you toward the oxide, sulfide, and native-element groups.

Streak is the color of a mineral's powder, revealed by dragging it across the unglazed streak plate. Streak is far more reliable than surface color: hematite can look silver or rust-red on the outside but always streaks red-brown, and pyrite streaks greenish-black, unlike real gold, which streaks gold.

Specific gravity is a mineral's density relative to water (numerically the same as density in g/cm³, since water is 1). Most common minerals sit around 2.5–3 (quartz is 2.65), but metallic minerals are much denser: galena is about 7.5, and gold is a remarkable 19.3. An experienced hand can feel that heft, and a simple mass-and-volume measurement confirms it — the surest way to tell genuine gold from fool's-gold pyrite (about 5.0).