Streak.

The color of the fine powder of a mineral is knows as its streak. The streak is frequently used in the identification of minerals, for, although the color of a mineral may vary between wide limits, the streak is usually constant. This property can be conveniently determined in the laboratiory by rubbing the mineral on a piece of unglazed porcelain, known as a streak plate.

'Dana's Manual of Minerology, sixteenth edition, 1955.