Reading a Rock: An Introduction to Rock and Mineral Identification
Identifying a rock or mineral by color alone is one of the most common mistakes beginning collectors make, since color is actually one of the least reliable identifying properties. Real identification relies on a handful of more consistent physical properties.
Hardness and the Mohs Scale
The Mohs hardness scale ranks minerals from 1 (talc, easily scratched by a fingernail) to 10 (diamond, the hardest known natural material), based on which minerals can scratch which others. Testing hardness against common reference objects is one of the most accessible identification tools available.
Luster: How Light Reflects Off the Surface
Luster describes how a mineral’s surface reflects light, and it is a more reliable identifying feature than color. Common categories include metallic, vitreous (glassy, like quartz), and dull or earthy. Two minerals of similar color can often be distinguished immediately by their luster.
Cleavage and Fracture
Cleavage describes breaking along smooth, flat, predictable planes determined by a mineral’s internal atomic structure, mica is a classic example, splitting into thin, flat sheets. Fracture describes breaking in a less predictable way, quartz typically fractures with a curved, glass-like break.
Crystal Habit
Crystal habit refers to the characteristic external shape a mineral tends to form when it crystallizes, cubic, hexagonal, needle-like, or many other forms. When visible, it can be one of the more distinctive identifying clues available.
Streak: A Property Color Can’t Fake
Streak is the color of a mineral’s powder, tested against an unglazed porcelain plate, and it is often different from the mineral’s surface color. Hematite, for example, can appear silvery, black, or reddish on the surface, but reliably produces a reddish-brown streak.
Read more about crystals, gemstones, and minerals, or return to the Steiner Collectables homepage.