The streak of a mineral is the colour of the powder produced when it is dragged across an unweathered surface. Unlike the apparent colour of a mineral, which for most minerals can vary considerably, the trail of finely ground powder generally has a more consistent characteristic colour and is therefore an important diagnostic tool in mineral identification. If no streak seems to be made, the mineral's streak is said to be white or colourless.
Streak is particularly important as a diagnostic for opaque and coloured materials. It is less useful for silicate minerals, most of which have a white streak or are too hard to powder easily.
The apparent colour of a mineral can vary widely 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 reflected by the specimen and thus change its apparent colour. When the specimen is dragged to produce a streak, however, it is broken into randomly oriented microscopic crystals, and small impurities do not greatly affect light absorption. The streak therefore reflects the essential colour of the mineral better than bulk colour does.
The surface across which the mineral is dragged is called a streak plate and is generally made of unglazed porcelain tile. In the absence of a streak plate, the unglazed underside of a porcelain bowl or vase, or the back of a glazed tile, will work. A streak is sometimes described more easily or accurately by comparing it with the streak made on another plate.
Because the trail results from the mineral being crushed into powder, a streak can only be made by minerals softer than the streak plate, around 7 on the Mohs scale. For harder minerals, the powder colour can be determined by filing or crushing a small sample and then rubbing it on a streak plate. Most harder minerals have an unhelpful white streak.
In practice, a fresh unweathered surface should be selected and drawn gently across the plate with enough force to produce a uniform powder. Excessive force may crush the plate or embed the mineral and lead to a false colour. The streak should be examined in natural or standard light, and powder should be distinguished from scratches on the plate.
Some minerals leave a streak similar to their natural colour, such as cinnabar, lazurite and native gold. Cinnabar has a red streak matching its red crystals, lazurite has a blue streak matching its blue mass, and native gold has a golden-yellow streak close to its metallic colour.
Other minerals leave surprising colours. Fluorite always has a white streak, although it can occur as purple, blue, yellow or green crystals, because the colour comes from colour centres and impurities and the random orientation of the powder removes the macroscopic colour effect. Hematite is black in appearance but leaves a red streak, which accounts for its name from the Greek haima, meaning blood. Galena can resemble hematite in appearance but is easily distinguished by its grey streak.
Pyrite is brassy yellow with a greenish-black to brownish-black streak, chalcopyrite is brassy with a greenish-black streak, magnetite is black with a black streak, limonite has a yellowish-brown streak, and rhodochrosite has a white streak despite pink crystals. These examples show that streak can separate minerals with similar appearance.
Streak is a basic test in mineral identification and is usually combined with hardness, lustre, cleavage, specific gravity, magnetism and colour. For opaque metallic minerals, streak is often more reliable than bulk colour, whereas for transparent or pale minerals its diagnostic value is lower.
Limitations include the difficulty of testing minerals harder than the plate, variation of streak colour with grain size, oxidation or mixtures, and contamination or staining of the plate. Streak should therefore be part of a combined identification rather than the sole criterion. Modern identification also uses X-ray diffraction, electron microprobe analysis and spectroscopy for a more accurate mineral identity.