​​Describing streak

The streak of a mineral is the colour of the mark it leaves when it is ground to a powder. Because the streak isn't always the same as the colour of the mineral we see when examining the mineral as a whole, this provides a useful and sometimes diagnostic feature of the mineral. ​The standard way to create a streak is to scratch it against a harder, matte surface (typically an unglazed ceramic tile). This is then simply examined in good but indirect light and its colour determined. 

It has to be admitted that this isn't always possible, because very hard minerals won't leave a mark (other than a scratch in the surface of the plate). With many softer minerals, however - those with a hardness of about 6.5 or less - a clear streak can usually be made.  For minerals harder than this, the trick is to grind them up in a mortar and pestle then smear the granules or powder onto a ceramic tile to observe their colour.

Another issue to be dealt with is that many minerals produce a colourless or white streak which is not visible on a white ceramic tile. In this case, you need to use a black or coloured plate and test the streak on that, in order to see if any colour is produced. If it isn't, and yet a powdery of flaky streak can still be detected, the streak may safely be called colourless.

As this indicates, streak testing is  extremely easy, and as explained in the panel to right is often more diagnostic than the body colour (which can vary within minerals). The main constraint is that it works best with non-silicate minerals, for the majority of silicates produce a similar white streak. 

​The table below lists more common minerals associated with different colour streaks. White streaks are omitted because they are so common, as are minerals with colourless streaks. The table thus excludes most silicate minerals, which typically produce a white streak.

To see a longer list, go to: www.minerals.net/StreakSearch.aspx and if you want to learn more ab out streak, see this informative webpage at geoscience.blog​

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The science of streak

The colour we see when we look at a mineral represents what we might think of as the body colour of the mineral, as viewed from the outside. And as explained on the page about colour, it is a function not of the mineral per se, but of the way light reflects off the surface of the crystal, and thus of its chemistry and atomic arrangement.

​A different appreciation of mineral colour might be obtained if, instead, we could look at it unaffected by its crystal structure. One way of doing this is to create a powder from the mineral and inspect that.

This is what we do when we describe the streak of a mineral. We get this simply by scratching the mineral on a hard, matte plate (usually the unglazed surface of a ceramic tile) and describe the colour of the mark (streak) it makes. Somewhat surprisingly, perhaps, this streak is often different from that we see in the body colour of the mineral (see photo above).

​The reasons for this discrepancy is relatively easy to understand. In its crystalline form, the colour we see is greatly influenced by the crystal structure, as well as any minor impurities in its composition. When we grind the mineral up into a powder, the crystal structure is disassembled and the impurities largely disappear into the mass of the powder. What we're left with is the 'true' colour of the matter that makes up the mineral.

One huge advantage of the streak is thus that, while the mineral may present itself in different colours, the streak will almost always be the same. over the colour the crystal offers, for unlike the crystal colour the streak is almost always the same. It's therefore a much more diagnostic property than the body colour.
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Common minerals listed by the colour of their streak. Note that white and colourless streaks have been omitted; most silicates not included in this list may be assumed to have a white streak