What is a Mineral's Streak?
Mineral Streak: The True Colour Beneath the Surface
In mineralogy, streak is the colour of a mineral in powdered form, which may be different from the mineral's exterior.
Although a streak test can be useful in helping to identify a mineral, it should only be used in conjunction with other characteristics, such as colour, lustre, and hardness.
Different minerals with the same colour can have the same colour streak, and many have a white streak.
A streak test is carried out by scraping the mineral across an unglazed porcelain plate. This should be done firmly, otherwise no powder will be produced. Sometimes a considerable amount of pressure is needed.
Porcelain is used because it's a hard, man-made material that's readily available and relatively cheap. On the Mohs scale of hardness, porcelain grades about 7, and only a few minerals are harder than that.
Another advantage is that porcelain is naturally white, so the colour of the powdered residue can be easily seen. Rough porcelain is used instead of smooth because it helps break down fragments in the mineral.
A streak test will not be effective for minerals harder than porcelain. These minerals are said to have either a white or colourless streak. They're also likely to scratch the plate, which in itself will help with the mineral's identification.
The only minerals harder than 7 on the Mohs scale are topaz, which is grade 8, corundum which is grade 9 and diamond which is 10.
Magnetite and hematite can look very similar, but the streak in magnetite is black, while in hematite it's blood-red or reddish-brown.
Although pyrite and gold can have a similar appearance, gold has a golden or yellow-coloured streak, while in pyrite, it's brownish-black.
Topaz occurs in a wide variety of colours and can also be colourless. When heated, it changes colour, so identifying topaz on sight alone can be tricky.
Topaz can sometimes be mistaken for some types of quartz, but it's a much harder mineral. In a streak test, it would easily scratch the porcelain plate, telling you it can only be topaz, corundum or diamond.
Those with experience in carrying out a streak test will often be able to identify a mineral based on how easily it produces the streak.
Article Photos
Pop-up photos: Topaz, diamond, magnetite, topaz - Courtesy of Stan Celestian. Corundum - Courtesy of James St. John.
