The streak of a mineral is the colour of its powder obtained by rubbing it against the streak plate, which is usually made of unglazed porcelain . To identify the colour of the mineral in powdered form, it should be rubbed hard enough against the streak plate. Because the amount of mineral obtained is really small, be careful not to rub it with your finger. To obtain a proper coloured powder, the mineral specimen should be clean, and without weathering or other alteration effects, because in some cases the presence of impurities may change its colour. The streak of a mineral generally differs from its colour. For example, pyrite and chalcopyrite look very much alike, except for their colour: the former is brass-yellow, while the latter is golden-yellow. But their streaks are different, the former being black and the latter greenish.