The color of a mineral in powdered form, called streak, is often useful in identification. A mineral’s streak is obtained by rubbing it across a streak plate (a piece of unglazed porcelain) and observing the color of the mark it leaves (Video). Although a mineral’s color may vary from sample to sample, its streak is usually consistent in color. (Note that not all minerals produce a streak when rubbed across a streak plate. Quartz, for example, is harder than a porcelain streak plate and therefore leaves no streak.)
Streak can also help distinguish between mineral with metallic luster and those with nonmetallic luster.
Metallic mineral generally have a dense, dark streak, whereas minerals with nonmetallic luster typically have a light-colored streak.