How streak works
Drag the mineral firmly across an unglazed white porcelain plate (typically the back of a bathroom tile). The line of powder that's left is the streak — the color of the mineral pulverized to powder. Body color is dominated by reflection and depends on grain size and surface; streak is intrinsic to the chemistry.
Classic streak separations
Hematite (any external color) → red-brown streak. Magnetite (looks like hematite) → black streak. Pyrite (gold cubes) → greenish-black streak. Chalcopyrite (similar gold color, often iridescent) → also greenish-black but darker and metallic. Galena → dark grey. Sphalerite → pale yellow to brown to black depending on iron content. Cinnabar → bright red.
Limits
Minerals harder than the porcelain (~6.5) won't streak — they just scratch the plate. Quartz, garnet, topaz, beryl, corundum: streak test impossible. Use other diagnostics. Also: streak only works on a CLEAN plate. Wipe between tests; a leftover red hematite streak will contaminate your next sample.