Why the test works
Surface color can change with tarnish, staining, inclusions, or weathering. Powdering many tiny grains reduces some of those surface effects.
Streak is most diagnostic for minerals that yield a colored powder. Many pale nonmetallic minerals produce a white or nearly colorless streak, so the result must be combined with other evidence.
How to perform the test
- 1
Choose a clean point
Use an unweathered edge that represents the mineral of interest, not the surrounding rock, rust coating, or soil.
- 2
Make one short pass
Pull the specimen across the rough porcelain for about one to two centimeters with moderate pressure.
- 3
Separate powder from a scratch
Colored powder is a streak. A gouge with little powder means the specimen may be harder than the plate.
- 4
View in neutral light
Describe the powder precisely—red-brown, yellow-brown, gray-black, green-black, white—rather than copying the specimen’s body color.
- 5
Confirm and clean
Repeat on a fresh spot if the first line crossed a coating or mixed grain. Clean reusable plates so old residue does not contaminate the next test.
How to read the result
Red to reddish-brown powder
Strong evidence for hematite or hematite-rich material, even when the specimen looks steel gray or black.
Yellow-brown to brown powder
Consistent with goethite and some hydrated iron-oxide mixtures.
Black powder with strong magnetism
Supports magnetite, but mixtures and other dark magnetic minerals require additional checks.
White or nearly colorless powder
Common among many nonmetallic minerals and rarely diagnostic by itself.
Plate is scratched with little powder
The specimen is likely harder than the plate; record the streak as unavailable rather than guessing.
Examples from the LithoMap guide library
Common mistakes
- Testing a rusty coating instead of the underlying mineral
- Calling a porcelain scratch a streak
- Using glazed tile, which lacks the required abrasive surface
- Letting residue from an earlier sample alter the color
- Assuming a white streak identifies a single mineral
Test safely
- Use minimal pressure and avoid breathing powder.
- Do not streak fibrous material, lead-bearing material, radioactive candidates, or unknown mine waste.
- Wash hands after testing and keep plates away from food-preparation surfaces.
- Only test material you may lawfully possess and alter.
This method supports preliminary field identification. It is not laboratory confirmation, appraisal, hazard clearance, proof of origin, or permission to collect or alter a specimen.
Frequently asked questions
Why is streak sometimes more useful than color?
Tarnish, staining, and trace impurities can change a specimen’s surface color. Powder color is often more stable, although it is not unique for every mineral.
Why did my specimen scratch the streak plate?
Unglazed porcelain is commonly around Mohs 6.5 to 7. A harder specimen may cut the plate instead of leaving enough powder for a meaningful streak.
Can I use the unglazed ring under a mug?
It may work for a rough check, but ceramics differ and the object can be damaged. A purpose-made clean streak plate is safer and more consistent.
Authoritative sources
LithoMap wrote this guide in original language using the physical-property standards and safety information supported by these sources.
- Minerals and their physical properties U.S. National Park Service
- Geology Lesson 2: Global Building Blocks U.S. National Park Service
- Kansas Rocks and Minerals: Mineral Identification Kansas Geological Survey
Editorial review: September 18, 2026.