The streak of a mineral is its color when it is ground into a fine powder. This may be done by crushing a mineral fragment, placing the powder on a sheet of white paper, and observing the color. A more common (and convenient) procedure is to rub the mineral firmly across a tile of unglazed white porcelain (called a streak plate) to produce a line of powder whose color is called the streak of the mineral. This property may be distinctive when the color of the mineral is different from that of its streak, since the streak varies only slightly from one specimen to another. Streak is most useful for the identification of dark-colored minerals such as metallic sulfides and oxides; its usefulness more limited when testing light-colored sulfates, carbonates, or silicates. Also, minerals having a hardness exceeding that of the streak plate (about 6.5) cannot be tested in this manner. 
The streak (color of a powdered mineral) is listed where it is a useful diagnostic parameter in identification. Streak is useful almost exclusively in the case of opacity metallic minerals. The powder of most transparent minerals is white or colorless.