Streak

Streak is the colour of a powdered mineral - what you see when a mineral sample is rubbed across an unglazed white porcelain tile called a streak plate, leaving a fine powder behind. ^[1] The rough surface of the tile abrades the mineral and produces a small amount of powder whose colour is easy to see against the white background. If a streak plate is not available, a knife blade, rock hammer, or similar implement can be used to produce a small amount of powder from an unknown mineral. ^[1]

Why Streak Is More Reliable Than Colour

The value of streak lies in its consistency. For most minerals, streak colour is quite consistent regardless of variations in the colour of the hand sample itself. ^[1] This matters because hand-sample colour can vary widely even within a single mineral species, and streak cuts through that variability by revealing the powder’s intrinsic optical properties rather than those of a polished or weathered surface.

Limitations

Streak is most useful for minerals with a metallic lustre, where streak colour tends to be vivid and distinctive. For nonmetallic minerals, however, streak colour is usually white, grey, or some other pale colour and is therefore not a particularly useful diagnostic property for distinguishing between them. ^[1] The streak test is most worth performing when the mineral has a metallic or submetallic lustre, or when there is reason to suspect it belongs to the smaller group of nonmetallic minerals with distinctively coloured streaks.

References

1. Nesse, W. D. (2017). Introduction to Mineralogy, 3rd ed. Oxford University Press.

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