What is a Mineral's Streak?

A Mineral's Streak May Help with Identification

In mineralogy streak is the colour of a mineral in powdered form.  The colour may be different from the exterior of the mineral.  A streak test can be used to help with the 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 such as 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 several 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.

Because porcelain is naturally white, the powder's colour 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 have either 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 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.  Pyrite and gold can also look similar.  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 quartz but it's much harder.  In a streak test, it would easily scratch the porcelain plate which would tell you it must 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.