The streak test

A mineral's colour depends on its impurities, on how weathered it is, on the light you look at it in. The colour of its powder depends far less on any of that. It is why mineralogists crush a little material before concluding — and why the streak test, which does the same thing in one gesture, is well worth the tile it asks for.

What you need

A plate of unglazed porcelain. In a mineralogy shop it is called a streak plate and costs a few euros. At home it is free:

• The back of a ceramic tile. The visible face is glazed, the back is not. Same material, same hardness.
• The foot of a cup or plate. The ring the piece rests on is left unglazed so it does not stick in the kiln.

Do not use a glazed surface: the glaze is smooth and vitrified, and it will take nothing off.

The gesture

Hold the plate flat on a table. Press the stone against it and pull firmly over three or four centimetres, once. Look at the mark in daylight.

Two precautions:

Clean the stone first. A film of oxide or mud leaves a streak of its own, and you would be reading the colour of the dirt.

Blow across the mark, or brush it with a finger. Loose dust is not a streak. What stays embedded in the grain of the porcelain is.

If the stone scratches the plate instead of leaving powder on it, stop: you have just learned that it is above 6.5 on the Mohs scale, which is a finding in its own right.

The streaks that settle a question

Mineral	Appearance	Streak
Haematite	black, steel grey or red	red-brown, always
Magnetite	metallic black	black
Pyrite	brass yellow	greenish black
Chalcopyrite	golden yellow, often iridescent	greenish black
Gold	yellow	yellow, shiny
Galena	lead grey, very heavy	lead grey
Limonite / goethite	brown, ochre	yellow-brown
Cinnabar	red	scarlet
Malachite	green	pale green
Sphalerite	resinous brown to black	pale brown to yellow

Three readings of that table are worth underlining.

Haematite is the textbook case. It turns up as black masses, as red nodules, as steel-grey flakes — three appearances with nothing in common. The streak is red-brown in all three. It is that test, and not the colour, that names it.

Streak separates gold from pyrite for good. One leaves a bright yellow powder, the other a greenish-black one. There is no possible ambiguity, where the bulk colour has misled people for as long as there have been prospectors — which is where "fool's gold" comes from, detailed on the pyrite page.

Magnetite and pyrite both leave black. Streak does not do everything: here the bulk colour and magnetism decide. A fridge magnet picks up magnetite, not pyrite.

Its limits

The streak test is worthless above 6.5, which rules out the whole quartz group — amethyst, agate, flint, jasper, chalcedony, citrine. Those minerals do not wear on the plate, they mark it.

It is also uninformative for many pale non-metallic minerals, whose streak is simply white: quartz, feldspar, calcite, gypsum and fluorite all leave white or colourless powder. A white streak says only one thing, and that is already something: this is not a metallic mineral.

In other words, streak is a specialist's test. It tells you almost nothing about a pale translucent stone, and it is decisive on a dark, heavy, shiny one. That is exactly where the eye fails hardest — which makes it the natural complement to hardness rather than its competitor.