streak, the colour of a mineral in its powdered form. It is usually obtained by rubbing the mineral on a hard, white surface, such as a tile of unglazed porcelain, so as to yield a line, or streak, of fine powder. The colour of the streak is usually constant for a given species of mineral, even though the mineral may vary considerably in colour as it occurs in the field. Indeed, the colour of a streak may differ considerably from the colour of the unpowdered mineral. Streak is one of the more useful diagnostic properties, since it is a quick way to distinguish between different species of minerals that are otherwise similar in appearance.