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What is Streak?

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Streak a Mineral in Powdered Form

In mineralogy a streak test can be used to help with the identification of certain minerals.  A mineral's streak is its colour in powdered form and may be different to the colour of the stone.

Gold has a golden or yellow coloured streak yet in the mineral pyrite which can look quite similar it's brownish black.  .

A streak test is carried out by scraping the mineral across an unglazed porcelain plate.  With harder minerals it needs to be done firmly and with pressure otherwise no powder will be produced.

Although a streak test can help with the identification of certain minerals, other characteristics and properties should also be looked at.  Different minerals with the same colour can have the same coloured streak and many 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 minerals harder than porcelain a scratch 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.

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.  It can sometimes be mistaken for quartz but a streak test would enable you 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 depending on how easy it is to produce the streak.

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