Introduction
Imagine you are handed an unlabelled fragment of an unknown mineral. No context, no hints. In a field geology kit you have: a fingernail, a copper coin, a piece of glass, a steel nail file, and a small tile of unglazed porcelain. That is all. Within two minutes, an experienced geologist can use those five objects to narrow the mineral down to a handful of candidates — and often identify it precisely.
This is not guesswork. It is the direct consequence of something we established in Lesson 1.2.1: every sample of a given mineral has the same crystalline lattice. The same lattice means the same bond geometry throughout the crystal. The same bond geometry means the same resistance to scratching, the same tendency to break in particular directions, and the same way the surface interacts with light — everywhere, every time, in every sample of that species on Earth. Identification is possible because consistency is built into the physics.
This lesson introduces the four main physical properties used to identify minerals in the field — hardness, streak, cleavage, and luster — along with the supporting properties that help when the main four are ambiguous. Together they form an identification toolkit that geologists have been refining for two centuries, and that still works as well today as it did when Friedrich Mohs published his hardness scale in 1812.
Key Terms
A mineral's resistance to being scratched, determined by the strength and geometry of bonds in its crystal lattice. Measured on the Mohs scale from 1 (softest — talc) to 10 (hardest — diamond). A harder mineral will always scratch a softer one.
The colour of a mineral when powdered — determined by dragging the mineral across an unglazed porcelain plate. Streak is more reliable than surface colour for identification because trace impurities can change a mineral's surface colour while leaving its streak unchanged.
The tendency of a mineral to break along flat, smooth planes in specific directions, controlled by planes of weaker chemical bonds running through the crystal lattice. Cleavage planes are reproducible: every break in that direction produces the same flat surface at the same angle.
The quality and character of light reflected from a mineral's surface — not its colour, but how the surface handles light. The two primary categories are metallic (shiny, opaque, like polished metal) and non-metallic (which includes vitreous/glassy, waxy, pearly, silky, and adamantine).