Why a mineral leaves a streak that is different in colour than it's appearance?
The property of streak is seen when a mineral is rubbed against a porcelain streak plate that is unglazed. A powder will be left behind and that produces the mineral's signature streak. Because a streak plate has a hardness of 6.5, the streak test will work on minerals less than 6.5 on Moh's scale. However, color is seen by the naked eye, when light either reflects off a mineral or is transmitted by a mineral, if that mineral is translucent. This can be different from the streak. Color is not a reliable means to identify a mineral. Fool's gold, or pyrite is a brassy yellow mineral, but when the streak test is performed, it leaves behind a black streak. Actual gold also looks yellow in color, and its streak is also yellow. It is a mineral's coating and structure that produces the outward color, however, the streak is really its true color. Some minerals may even have trace elements in them, that could affect the color. Therefore, the streak test is more reliable for identification of a mineral than its color.