The streak (also called "powder color") of a mineral is the color of the powder produced when it is dragged across an unweathered surface. This trail's color generally is much more consistent than that of its parent mineral and therefore is an important mineral identification tool. If no streak appears, then the mineral's streak is said to be white or colorless. Streak is especially important for identifying opaque and colored materials and is less useful for silicate minerals, most whereof have a white streak and are too hard to easily powder.
The apparent color of a mineral can greatly vary because of trace impurities or a disturbed macroscopic crystal structure. Small amounts of an impurity that strongly absorbs a particular wavelength can radically change the wavelengths of light that the specimen reflects, thereby changing the apparent color. Dragging the specimen to produce a streak breaks into randomly oriented microscopic crystals, reducing the small impurities' effect.
The surface whereover the mineral is dragged is called a "streak plate" and generally is made from unglazed porcelain tile. The unglazed underside of a porcelain bowl or vase or the back of a glazed tile also works. Sometimes a streak is more easily or accurately described by comparing it with the "streak" made by another streak plate.
Because the trail left behind results from the mineral being crushed into powder, a streak can only be made of minerals softer than the streak plate, around 7 on the Mohs scale of mineral hardness. The color of harder minerals' powder can be determined by filing or crushing with a hammer a small sample, which is then usually rubbed on a streak plate. Most harder minerals have an unhelpful white streak.
Such minerals as cinnabar and lazurite leave a streak that resembles their natural color. Other minerals leave surprising colors: fluorite always has a white streak and can appear in purple, blue, yellow, or green crystals. Hematite, black, leaves a red streak that accounts for its name, which comes from the Greek word "haima," meaning "blood." Galena, which can superficially resemble hematite, is easily distinguished by its gray streak.
- Bishop, A.C.; Woolley, A.R.; Hamilton, W.R. (1999). Cambridge Guide to Minerals, Rocks and Fossils (2nd ed.). Cambridge: Cambridge University Press. pp. 12–13.
- Holden, Martin (1991). The Encyclopedia of Gemstones and Minerals. New York: Facts on File. p. 251. ISBN 1-56799-949-2.
- Schumann, Walter (1992). Minerals of the World. New York: Sterling Publishing. pp. 18–16. ISBN 0-00-219909-2.
- Physical Characteristics of Minerals, at Introduction to Mineralogy by Andrea Bangert
- What is Streak? from the Mineral Gallery
- Streak at CSU Pomona Geology