Streak (mineralogy)

Streak is the color of a mineral's powder when it is dragged across a streak plate. This plate is typically a piece of unglazed porcelain. Streak is a more reliable property for mineral identification than the apparent color of the mineral sample itself, which can vary due to impurities or surface alterations.

The streak color is determined by the mineral's chemical composition and is generally consistent for a particular mineral, even if the hand-specimen color varies. For example, hematite can appear black, brown, or red as a bulk sample, but its streak is always a characteristic red-brown.

To obtain a streak, the mineral is rubbed firmly across the streak plate. The powdered material left behind is the streak, and its color is then observed in good light. If the mineral is harder than the streak plate (Mohs hardness of approximately 7), no streak will be produced because the mineral will scratch the plate rather than leave powder. In these cases, the mineral is described as having "no streak." For soft minerals, the streak test is a valuable diagnostic tool.