How to Identify Minerals by Physical Properties

Mineral identification is a systematic process that examines multiple physical properties to narrow down the possibilities. No single property is usually sufficient for identification; instead, the combination of several properties leads to a positive identification.

Start with color, but be aware that many minerals come in multiple colors (quartz, tourmaline, fluorite), while different minerals can share the same color. Color is the least reliable identification property.

Streak — the color of the mineral's powder — is far more reliable. While pyrite and gold may both appear yellow, pyrite produces a greenish-black streak while gold streaks golden yellow. Use an unglazed porcelain tile for streak testing.

Luster describes how light reflects from the mineral's surface. The two main categories are metallic (like polished metal) and non-metallic. Non-metallic lusters include vitreous (glassy, like quartz), adamantine (brilliant, like diamond), pearly, silky, waxy, resinous, and dull.

Hardness testing using the Mohs scale is one of the most useful diagnostic properties. Combined with streak, it can quickly narrow the possibilities.

Cleavage — the tendency to break along flat surfaces — is determined by the crystal structure. The number of cleavage directions, their angles, and quality help distinguish minerals. Micas have one perfect cleavage, feldspars have two at nearly 90 degrees, and calcite has three forming rhombohedra.

Crystal form, when visible, is highly diagnostic. The shapes of crystal faces, their angles, and twinning patterns are direct reflections of the internal crystal structure.