Mineral streak explained

The streak (powder color) of a mineral is the color of the powder that is produced when it’s dragged across an un-weathered surface, like unglazed porcelain. Unlike the obvious color of a mineral, which in most minerals can vary considerably, the trail made  up if the finely ground powder has a consistent color, making it an important diagnostic tool in rock and mineral identification. If no streak seems to be made, the mineral's streak is considered to be white or colorless. Streak can be particularly important as a diagnostic for opaque and solid colored material, and less useful for stuff like silicate minerals, most of which will have a white streak or sometimes too hard to powder easily.  When a specimen is dragged to produce a streak, it is broken into randomly oriented microscopic crystals, so small impurities don’t much affect the absorption of light, which is what can make solid minerals appear different colors on the same specimen.  The surface across which the mineral is dragged is called a "streak plate," usually made of unglazed porcelain tile. The unglazed underside of a porcelain bowl or vase or the back of a glazed tile will work. Sometimes a streak is more easily or accurately described by comparing it with the "streak" made by another streak plate. In case of harder minerals, the color of the powder can be determined by filing or crushing with a hammer a small sample, which is then usually rubbed on a streak plate. Most minerals that are harder have an unhelpful white streak.  Some minerals leave a streak similar to their natural color, like lazurite. Other minerals leave surprising colors, like fluorite, which always has a white streak, although it appears in purple, blue, yellow, or green crystals. Hematite, which is black, leaves a red streak which accounts for its name, which comes from the Greek word "haima," which means "blood." Galena, which is similar in appearance to hematite, is easily distinguished by its gray streak.