Streak is a Mineral in Powdered Form
In mineralogy a streak test is used to help with the identification of a mineral. Streak refers to a mineral in powdered form and the colour can often be different to the outer surface of the stone. The colour of hematite for example varies from black to silver grey, brown to reddish brown and may also be red but the streak is either blood red or reddish brown. Pyrite also known as fool's gold has a brassy yellow metallic surface but its streak is brownish black.
A streak test is carried out by scraping the mineral across an unglazed porcelain plate. With harder minerals this must to be done firmly and with pressure otherwise no powder will be produced. A porcelain plate is used because porcelain grades 7 on Mohs scale of mineral hardness which is harder than most minerals.
Mohs scale measures the hardness of a mineral by means of scratch resistance. Each mineral is given a number between 1 and 10 depending on whether it can scratch or be scratched by a different mineral. A mineral graded 4 will be able to scratch one that's graded 3 and a mineral that's graded 5 will be able to scratch one that's graded 4. Those with the same grade will be able to scratch each other. With porcelain being 7 it's similar to quartz which is the index mineral for this grade of hardness.
A streak test is particularly useful for minerals that occur in different colours. Fluorite can be red, pink, white, purple, blue, green, yellow, black or even colourless and colour zoning is also common which means different colours can occur within the same crystal. Irrespective of the exterior colour of fluorite its streak is always white.
Although a streak test is useful when trying to identify a mineral it should only be used in conjunction with other findings.
For minerals harder than porcelain a scratch test will not be effective and these materials are said to have a white or colourless streak. They're also likely to scratch the plate which in itself will help with identification. The only minerals harder than 7 on Mohs scale are topaz which grades 8, corundum which grades 9 and diamond which is 10. An inspection of other characteristics and properties should make identification of these minerals quite straight forward.
Topaz occurs in different colours, can be colourless and can change colour when heated or irradiated. It's sometimes mistaken for quartz but a streak test would quickly reveal that it's harder than the porcelain plate which confirms it can only be topaz, corundum or diamond.
Those with experience in carrying out a streak test will often be able to identify a substance or its hardness depending on how easy it is to produce the streak.