Gleamite

Streak test, the working method

By The Gleamite desk, Identification research · updated

The streak test reads a mineral's powder color by dragging it across unglazed porcelain, and powder is steadier than surface: hematite streaks red-brown whether the stone is mirror-metallic or dull earth. The unglazed back of a tile or a mug's bottom rim works at home, and the test fails only above porcelain hardness, near Mohs 7.

Running the test

  1. Step 1Find unglazed porcelainA streak plate if you own one; otherwise the unglazed back of a ceramic tile or the rough foot-ring on a mug bottom. Glazed surfaces are too hard and too smooth to shed honest powder.
  2. Step 2Drag a corner firmlyOne confident inch with a point or edge of the specimen. Light strokes smear; firm pressure powders. Use an inconspicuous corner — the test grinds off material.
  3. Step 3Read and wipeJudge the line in daylight against the white plate, then wipe clean. Residue from the last test is the easiest way to misread the next one.

One honest limit governs everything: the plate itself sits near 7 on the Mohs scale. Minerals at quartz hardness and above scratch the porcelain instead of powdering, leaving a pale gouge of plate material — a non-result that is itself a verdict, since it clears the specimen of every softer metallic impostor at once. That asymmetry is the test’s quiet second gift.

The streak chart

Streak colorMineralsWhat it settles
Golden yellowGoldThe only yellow streak among the brassy suspects
Greenish blackPyrite, chalcopyriteEnds every fool’s gold debate
BlackMagnetite, graphiteGraphite also writes on paper
Red brownHematiteRegardless of metallic or earthy surface
Yellow brownGoethite, limoniteThe color behind stained hillsides
Lead grayGalenaPaired with extreme weight
White or noneQuartz family, feldspar, calcite, harder gemsSoftness checks take over from here

The single most consequential row is the greenish-black one: it is the difference between a story and a windfall, run in full — shade, dent, then streak — in how to identify gold in rocks. The red-brown row does the same veto work for meteorite hopefuls, since hematite and magnetite are the classic heavy magnetic meteorwrongs, per how to identify a meteorite.

Where streak fits among the tests

Streak is the third test, not the first. Luster sorts candidates at a glance and hardness brackets them with household tools — the full test-order logic lives in the mineral identification chart, where streak is one column among five. Its specialty is the metallic and earthy dark minerals, where surface color misleads most and powder color decides fastest; the deep-red finds where the rule shines are worked through in red rock identification.

Two care notes close the method. The test sacrifices a corner of the specimen — on a display-quality crystal, streak an inconspicuous edge or skip the test entirely. And a small set of colorful minerals are not worth powdering at all: anything suspected of being cinnabar — mercury sulfide with a vivid red streak — stays untested and unhandled, because no identification is worth grinding a toxic mineral by hand.

Questions people ask

How do you do a streak test at home?
Find unglazed porcelain — the back of a ceramic tile or the rough ring on a mug's bottom — press a corner of the mineral firmly against it, and drag an inch. Read the powder line's color in good light, then wipe the plate clean between tests. If the mineral is harder than the plate, it scratches porcelain instead and leaves no honest streak.
Why is streak more reliable than mineral color?
Surface color varies with impurities, weathering, tarnish and grain size, but finely powdered mineral shows the substance's own color. Every quartz variety — amethyst to smoky — streaks the same white, while hematite streaks the same red-brown whether the specimen is metallic silver or dull rust. The powder ignores the costume.
What mineral leaves a black streak?
The usual answer is magnetite, with graphite close behind — graphite also writes on paper. Pyrite and chalcopyrite pull greenish black despite their brassy gold surfaces, which is the streak test's most famous verdict. A dark heavy stone with a clean black streak and strong magnet response is magnetite until proven otherwise.
What does it mean if there is no streak at all?
Usually that the mineral is harder than the plate — quartz, garnet, topaz and corundum powder the porcelain instead, leaving a white scratch of plate material. That non-result is itself information: a colorless streak from a hard mineral rules out every soft metallic impostor in one drag.
The Gleamite deskWrites every identification chart on this site from named mineralogical properties — Mohs values, streak colors, specific gravity — sourced from USGS and standard references on a stated date, and runs the same hardness, streak and vinegar checks it tells readers to run.
Sources