What is a Mineral's Streak?
Streak Can Help Identify a Mineral
In mineralogy streak is the colour of a mineral in powdered form. The colour may be different to the exterior of the mineral. A streak test can be used to help with identification of some minerals.
The problem with a streak test is an incorrect result can be achieved for a variety of reasons. It should therefore only be used in conjunction with other characteristics and properties. These can include colour, lustre and hardness.
Different minerals with the same colour can have the same colour streak. Many have a white streak.
A streak test is carried out by scraping the mineral across an unglazed porcelain plate. This must be done firmly otherwise no powder will be produced. Sometimes a considerable amount of pressure is needed.
Porcelain is used for a number of reasons. It's a hard man-made material that's readily available and relatively cheap. On Mohs scale of hardness it grades about 7, not many minerals are harder than that. With porcelain being white the colour of the powder can be easily seen. Rough porcelain is used instead of smooth because it helps break down fragments in the mineral.
For minerals harder than porcelain a streak test will not be effective. 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.
Magnetite and hematite can look very similar but the streak in magnetite is black whereas in hematite it's blood red or reddish brown. Pyrite and gold can also look similar. Gold has a golden or yellow coloured streak whilst 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 this mineral on sight alone can be tricky. Topaz can sometimes be mistaken for quartz but is much harder. In a streak test it would easily scratch the porcelain plate which would tell 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 depending on how easy it is to produce the streak.