How to identify a mineral by its streak
The streak test is a simple, reliable way to help identify a mineral by the color of its powdered form. It works for many common minerals and can be done with minimal equipment; you can record results with a photo or scanner and compare them to reference samples to check recognition and worth.
Quick facts
- Plate
- Streak plate: unglazed porcelain
- Hematite
- Hematite streak: reddish brown
What the streak tells you
A streak is the color of a mineral in powdered form when it is rubbed against a hard, rough surface. The streak can differ from the color of the whole crystal, gem or gemstone because impurities and surface weathering alter the outer color. Streak is especially useful when a finder is working with metallic-looking minerals or when the specimen's surface is coated or altered. Use the streak result alongside other properties to judge identification and potential value or worth.
Tools and safety
Use a clean, flat surface called a Streak plate: unglazed porcelain for the test. Hold the specimen firmly and wear eye protection; small particles can flake off. Do not use your hand as the plate. You can document the result with a photo or a scanner to compare later. Keep separate plates for different mineral groups to avoid cross-contamination.
Step-by-step streak test
1) Place the streak plate on a stable surface. 2) Hold the specimen with a firm grip and press one edge against the plate. 3) Drag the specimen across the plate with steady pressure to produce a streak of powder. 4) Inspect the powder color under white light. 5) Record the result with a photo or scanner for later comparison. If no streak appears, the mineral may be harder than the plate; try a softer edge of the specimen or use other tests.
Common streak colors and an example
Streak colors can be diagnostic: for example, Hematite streak: reddish brown is a characteristic result that helps separate hematite from other dark minerals that may look similar on the surface. Remember that some metallic minerals leave a dark or colored streak even when the sample looks black or silvery.
Pitfalls and practical tips
Do not rely on streak alone. Hard minerals can scratch the plate without leaving a true powder streak. Weathered surfaces, coatings, or encrustations can mask the true streak color. Always test a fresh edge of the specimen when possible. Keep a small notebook or digital folder with photos to track recognition and to assess how streak results affect the specimen's apparent value or worth.
Documenting results and using apps
After the streak test, take a clear photo or use a scanner to save the powdered streak and the tested specimen. Use those images when consulting field guides or online resources for recognition. For additional help, you can use Rock Identifier to compare your photo to reference images and supporting data.
Common questions
- Will the streak test damage my specimen?
- The test removes a small amount of material from the specimen and can alter a fresh edge. For rare, valuable, or cut gems and gemstones do not perform a streak test. For common rocks and minerals, test a small, less visible area.
- Can I use any white tile instead of a streak plate?
- No. Use a proper Streak plate: unglazed porcelain. Glazed tiles or smooth surfaces will not produce a reliable powdered streak and can give misleading colors.
- What if the specimen is harder than the streak plate?
- If the mineral is harder than the streak plate it may not leave a streak. In that case rely on other tests such as hardness, crystal habit, luster, and specific gravity for identification.