What is Streak? Streak is the color of a mineral’s powdered form, checked by rubbing it on an unglazed porcelain streak plate. Streak can be more consistent than surface color, especially for metallic minerals. Use light pressure and a fresh corner; a slick, weathered face often gives a misleading mark. Streak is most useful when you combine it with hardness and magnetism.
For Streak, keep a small streak plate clean because metallic powders can coat it and interfere with later tests. Some hard silicates won’t leave a streak and may only scratch the plate; that is still useful information—write it down as “scratches plate, no streak.” Test a clean spot on the specimen; dirt, iron oxide, or clay can color the mark. In your notes, pair streak color with luster and hardness so the identification value survives cleaning and handling.
Streak vs. surface color Surface color can be stained or altered. Streak is the powder color and often stays consistent even when the surface looks different.
Streak vs. dirt or rust residue Clay and iron oxides can smear onto the plate. Clean the sample and re-test on a fresh edge.
Streak vs. “no streak means it’s not that mineral” Some minerals simply won’t powder on porcelain. Record “scratches plate” instead of forcing a streak color.
Streak vs. metallic appearance Metallic-looking minerals can have very different streaks. Use streak to confirm, not appearance alone.