Streak | A Mineral in Powdered Form
In mineralogy a streak test can be used to help with the identification of a mineral. A mineral's streak is its colour in powdered form and is not always the same colour as the stone. The streak in gold is golden or yellow yet in pyrite which can look quite similar its brownish black. .
A streak test is carried out by scraping a mineral across an unglazed porcelain plate. With harder minerals it needs to be done firmly and with pressure otherwise no powder will be produced.
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 that grades 4 will be able to scratch one that grades 3 and one that's graded 5 will be able to scratch one that's graded 4. Those with the same grade can scratch each other. Quartz grades 7 and porcelain is usually very similar.
The colour of the streak can help with the identification of certain minerals but other characteristics and properties should also be inspected. Different minerals with the same colour can have the same coloured streak and many minerals have a white streak. Magnetite and hematite can look very similar but the streak in magnetite is black whereas in hematite it's blood red or reddish brown.
For materials harder than porcelain a scratch test will not be effective so these minerals are said to either 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.
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. This gemstone is sometimes mistaken for quartz but by doing a streak test you would quickly be able to tell one from the other. Topaz is much harder than quartz so would easily scratch the porcelain plate. That alone would confirm it could only be topaz, corundum or diamond.
Those with experience in carrying out a streak test will often be able to identify a mineral or its hardness depending on how easy it is to produce the streak.