The Streak Test: How to Identify Minerals by Their Streak
The streak test is one of the simplest and most reliable tools in mineral identification. "Streak" is the color of a mineral in its powdered form, found by dragging the mineral across a piece of unglazed porcelain and looking at the fine line of powder it leaves behind. That powder color is often very different from the color of the solid specimen in your hand — and, crucially, it is far more consistent from sample to sample.
Why does the powder tell a truer story than the surface? The color of a whole mineral can be thrown off by tiny amounts of impurities, by weathering, and by a thin film of tarnish on metallic specimens. Quartz, for example, comes in dozens of colors yet is essentially the same mineral. When you crush a mineral to powder, light scatters off countless tiny grains instead of one shiny face, which cancels out a lot of that surface variation and reveals a more dependable, characteristic color. That is what makes streak such a quick and valuable identification clue, especially for darker and metallic minerals.
How to Do a Streak Test
All you need is a streak plate: a small square of unglazed (matte, unfinished) porcelain. The unglazed bottom of a ceramic tile or the rough underside of a kitchen mug works just as well. A streak plate has a hardness of about 7 on the Mohs scale, which matters for the test's main limitation, described below.
Work in good light over a clean, light-colored surface, and follow these steps:
- Pick up a fresh, clean edge or corner of the mineral specimen. Avoid weathered or dirty surfaces, which can give a false color.
- Press the mineral firmly against the unglazed porcelain plate and drag it across the surface in a single, firm stroke.
- Lift the specimen and look at the line of powder left on the plate. That colored mark is the streak.
- Compare what you see against a streak-color chart. If the result is faint, repeat with slightly more pressure or stack several strokes in the same spot.
- Wipe the plate clean (a damp cloth or a little water removes most streaks) before testing the next mineral.
The Key Limitation: Hard Minerals Leave No Streak
A streak plate is only about hardness 7. Any mineral harder than the plate will not leave a streak — instead of shedding powder, the mineral scratches the porcelain and may leave a thin white mark that is actually powdered plate, not powdered mineral. That is why common hard minerals such as quartz (hardness 7), topaz (8), beryl, and corundum either leave no streak or a misleading white one.
Because of this, the streak test is most useful for softer minerals and for metallic minerals like hematite, pyrite, galena, and magnetite, which are soft enough to powder readily. If a mineral refuses to leave any streak, that absence is itself a clue: it tells you the specimen is harder than 7, which already narrows the field. Pair streak with a Mohs hardness test and the two results together rule out a great many candidates.
Streak Colors of Common Minerals
The chart below lists the characteristic streak colors of widely encountered minerals. Note how often the streak differs from the specimen's outward appearance — that contrast is exactly what makes the test so useful.
| Mineral | Streak color |
|---|---|
| Hematite | Reddish-brown |
| Magnetite | Black |
| Pyrite | Greenish-black to brownish-black |
| Galena | Lead-gray |
| Limonite / Goethite | Yellow-brown |
| Chromite | Brown |
| Cinnabar | Red (scarlet) |
| Graphite | Gray to black |
| Chalcopyrite | Greenish-black |
| Gold | Golden-yellow |
| Quartz and most silicates | White or colorless |
| Calcite and most carbonates | White |
Why Surface Color Fools People
The classic cautionary example is hematite. In specimen form it can look metallic silver-gray, steely, or even black, and beginners are often surprised that such a dark, shiny stone leaves a distinctly red to reddish-brown streak every time. That rusty powder is a dead giveaway: whatever the surface looks like, a reddish-brown streak points straight to hematite (the same iron oxide responsible for the red color of rust and much red soil).
Pyrite versus gold is the other textbook case — the original "fool's gold" trap. Pyrite and gold can both gleam with a brassy, yellow metallic luster, so surface color alone fools a lot of people. Their streaks settle the question instantly. Pyrite leaves a greenish-black to brownish-black streak, while real gold leaves a golden-yellow streak. A shiny yellow mineral that streaks dark green-black is pyrite, not gold. These examples show the core lesson of the streak test: trust the powder, not the polish.
Frequently asked questions
What is a streak test?
A streak test is a mineral identification method that reveals the color of a mineral in powdered form. You drag the mineral across a piece of unglazed porcelain (a streak plate) and observe the colored line of powder it leaves behind. This powder color, called the streak, is often more reliable for identification than the color of the whole specimen, because surface color can be altered by impurities, weathering, and tarnish.
What do I need to do a streak test, and can I use a regular tile?
You need an unglazed (matte, unfinished) piece of porcelain. A purpose-made streak plate is ideal, but the unglazed underside of a ceramic floor or bathroom tile works perfectly, as does the rough bottom ring of a coffee mug. The key is that the surface must be unglazed: a glossy, glazed tile is too smooth and slick to abrade the mineral and will not produce a streak.
Why doesn't quartz leave a streak?
Quartz has a Mohs hardness of about 7, the same as a porcelain streak plate. Because quartz is as hard as (or harder than) the plate, it scratches the porcelain instead of crumbling into powder, so it leaves no true streak — sometimes just a faint white mark that is actually powdered plate. This is why the streak test works best on softer and metallic minerals; if a mineral leaves no streak, that tells you it is harder than 7.
What color is hematite's streak?
Hematite has a reddish-brown streak, and it shows this color consistently no matter what the specimen looks like on the outside. This is striking because hematite often appears metallic silver-gray, steely, or black in hand. The reddish-brown powder is one of the most useful identification clues in mineralogy — the same iron oxide gives rust and red soil their color — and it reliably distinguishes hematite from similar-looking dark, metallic minerals.
Is streak color more reliable than a mineral's surface color?
For many minerals, yes. A mineral's surface color can vary widely because of trace impurities, weathering, and tarnish — quartz alone comes in many colors. Powdering the mineral averages out those surface effects and reveals a more consistent, characteristic color. Streak is especially dependable for dark and metallic minerals. It is most powerful when combined with other tests such as Mohs hardness, luster, and crystal habit, rather than used on its own.
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Last updated 2026-06-25. Identification guidance is educational — confirm important results with the tests described or a qualified expert.