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The streak color of a mineral is the color of the mark it leaves when rubbed against a rough surface, such as an unglazed porcelain plate (known as a streak plate). It is characteristic of a mineral and therefore an important identification tool. In idiochromatic minerals, the streak usually matches the mineral's color. Allochromatic minerals often have a streak color that does not correspond to their intrinsic color. Visually similar minerals can often be distinguished by their streak color. Minerals harder than the streak plate (hardness >6) do not leave a true streak but merely scratch the plate. If you are unsure about the streak color, comparing it to the streak color of a previously identified mineral specimen is often helpful.
Figure 6: Streak colors of two different minerals on typical test plates
( Ra'ike (see also: de:User:Ra'ike) , CC BY-SA 3.0 , via Wikimedia Commons)