The porcelain streak test

The test that unmasks “fool's gold”

The streak test is one of the founding gestures of mineralogy, and one of the most disconcerting for beginners. The principle: rub the mineral on a piece of unglazed porcelain and observe the colour of the powder trail left behind. That colour is very often different from the mineral's own — and infinitely more reliable.

Why the powder does not lie

A mineral's outer colour depends on impurities, surface weathering, treatments, lighting. The colour of its powder, by contrast, is constant for a given species, because it reflects the mineral's deep composition, free of surface effects and light reflection.

The most spectacular example is hematite: depending on the specimen it may be steel grey, metallic black or reddish — but its streak is always red-brown. That is its absolute signature; its name comes from the Greek haima, blood. Another textbook case: pyrite, golden and shiny as gold, leaves a greenish-black streak — whereas real gold leaves a golden-yellow one. Generations of prospectors were saved from ruin by a test that costs nothing and takes two seconds.

The method, step by step

You need a streak plate — a few pounds from a mineralogy supplier — or simply the unglazed underside of a mug or a piece of tile: the matte, rough part. Rub an edge of the mineral firmly across one or two centimetres, blow away the excess, and look at the colour.

The two limits to know

First, the test only works if the mineral is softer than porcelain (about 6.5 to 7 on the Mohs scale). A sapphire or a diamond would scratch the plate instead of leaving powder — which is, in itself, information about hardness.

Second, the test slightly damages the point of contact. So never perform it on a cut gem or a fine collection crystal: keep it for rough stones, on the base or a hidden area. Combined with the Mohs scale, the streak test gives the amateur the two pillars of field mineral identification.