Rock IdentifierLuster, Streak, Cleavage & Fracture
Key takeaways
- Luster describes how a surface reflects light — start with metallic vs non-metallic.
- Streak is the powder color and is often more reliable than surface color.
- Cleavage (flat planes) vs fracture (irregular breaks) reveals atomic structure.
Luster: the character of the shine
Luster is how light interacts with a mineral's surface. The first question is metallic or not. Beyond that, non-metallic lusters have names worth learning: vitreous (glassy, like quartz), adamantine (brilliant, like diamond), resinous (like amber), pearly (like the inside of a shell), silky (fibrous minerals), greasy, and dull or earthy. Naming the luster quickly splits your candidates.
Streak: the honest color
Drag the mineral across unglazed porcelain and read the powder. Streak strips away surface staining and tarnish, so it's far steadier than the color you see. Golden pyrite leaves a greenish-black streak; red-to-black hematite always streaks reddish-brown. Streak only works for minerals softer than the tile (~7), so quartz and harder simply won't mark it.
Cleavage: breaking along planes
Cleavage is the tendency to split along flat surfaces where atomic bonds are weakest. Describe it by the number of directions and the angles: mica has one perfect cleavage (peels into sheets), feldspar has two at nearly 90°, halite and galena have three at 90° (cubes), and calcite has three not at 90° (rhombs). Consistent flat faces on broken pieces are a strong clue.
Fracture: breaking without planes
When a mineral lacks cleavage, it fractures. The classic is conchoidal fracture — smooth, curved, shell-like surfaces seen in quartz and obsidian. Other fractures are described as uneven, splintery, hackly (jagged, like broken metal), or earthy. Whether a specimen cleaves or fractures, and how, is one of the most structural — and useful — things you can observe.
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