Streak Test Guide

Interactive guide to using streak plates for mineral identification. Shows expected streak colors for common minerals and helps narrow down possibilities.

Identification

Minerals with this streak

How to Use

  1. 1
    Prepare a clean streak plate and fresh specimen surface

    Use an unglazed porcelain streak plate with a Mohs hardness of approximately 6.5. Ensure the plate is clean and dry, and expose a fresh, unweathered surface on the specimen by chipping if needed, as oxidation rinds and coatings can produce misleading streak colors.

  2. 2
    Draw a firm stroke across the streak plate

    Hold the mineral at an acute angle to the plate surface and drag it firmly across approximately 2–3 cm. Apply consistent moderate pressure—too light a stroke may not leave sufficient powder, while excessive pressure may grind the plate itself and contaminate the streak with porcelain powder (which appears white).

  3. 3
    Interpret the streak color and match to candidates

    Observe the powder color under natural light, noting both the primary color and any secondary hue. Compare the streak to the reference chart to identify candidate minerals. Remember that streak works only for minerals softer than 6.5; minerals harder than the streak plate (quartz, topaz, corundum) leave no streak and must be identified by other methods.

About

The streak test is one of the simplest and most reliable mineral identification techniques, exploiting the fact that powdering a mineral eliminates the optical effects—surface reflectance, thin-film interference, scattering—that make surface color variable and unreliable. The test has been described in systematic mineralogy texts since at least the 18th century and remains a standard first-line diagnostic tool in introductory geology and advanced field mineralogy.

The streak plate—unglazed white porcelain with a Mohs hardness near 6.5—provides a consistent abrasive surface that reduces minerals softer than 6.5 to fine powder without contributing significant plate material to the streak. The plate color (white to light gray) provides a neutral background against which subtle streak colors are most visible. Different streak plate textures and compositions can produce minor variations in apparent streak color, which is why reference collections of verified mineral streaks provide more accurate comparison standards than color charts alone.

For sulfide and oxide minerals in particular, streak testing complements luster and hardness tests to provide rapid, high-confidence identifications. The streak of hematite, limonite, cinnabar, malachite, azurite, and many other economically important ore minerals is sufficiently distinctive that experienced geologists can make field identifications in seconds. Modern mineralogical practice combines streak with portable X-ray fluorescence spectroscopy and Raman spectroscopy for ambiguous specimens, but streak testing retains its place as an immediate, cost-free diagnostic that requires no instrumentation.

FAQ

Why is streak more reliable than surface color for identification?
Surface color depends on both the true mineral pigment and on surface optical effects including thin-film interference, scattering from surface roughness, and alteration products. Streak eliminates these surface artifacts by reducing the mineral to a fine powder in which particle size is smaller than visible light wavelengths, preventing interference effects. Hematite (Fe₂O₃) illustrates the principle dramatically: massive specular hematite has a metallic silver-gray or black surface, botryoidal hematite is often red-brown, yet both always produce a characteristic red-brown streak because the powder exposes the true iron oxide pigment. The constancy of streak across varieties is particularly useful for metallic minerals in the oxide and sulfide groups.
What streak colors are most diagnostic for common metallic minerals?
Streak colors for metallic minerals follow recognizable patterns tied to chemical composition. Sulfide minerals typically produce dark gray to black streaks: galena (lead-gray), pyrite (greenish-black), chalcopyrite (black), and bornite (grayish-black) all streak darker than their surface colors. Iron oxide minerals produce characteristic red-brown (hematite) to yellow-brown (goethite, limonite) streaks reflecting ferric iron coloration. Molybdenite produces a distinctive greenish-gray streak on paper rather than porcelain, and its soft, greasy feel accompanies the test. Cinnabar (HgS) produces a scarlet streak matching its surface color. These patterns allow rapid discrimination between similarly colored metallic minerals.
How do you streak a mineral harder than the streak plate?
Minerals with Mohs hardness above 6.5—including quartz, feldspar, topaz, corundum, and diamond—cannot leave a streak on a standard porcelain streak plate because they abrade the plate surface rather than depositing powder themselves. For these minerals, the streak test is simply inapplicable, and identification relies on other properties: hardness bracket testing, luster, cleavage geometry, specific gravity, and crystal habit. A practical workaround is to powder the mineral using a steel mortar and pestle and observe the powder color directly, but this approach destroys specimen material and is rarely used in field identification. The inability to produce a streak is itself useful information: if a suspected mineral fails to streak a porcelain plate, its hardness exceeds 6.5.
Can fluorescence testing be combined with streak testing?
Fluorescence and streak tests address different mineral properties and are fully complementary, though they are conducted separately. Streak is conducted under visible light to assess pigment color. Fluorescence is assessed under UV light (both shortwave at 254 nm and longwave at 365 nm) in a darkened environment. Some minerals produce streaks that are themselves fluorescent: calcite streaks white on a plate but glows red-orange under UV. Scheelite (calcium tungstate) produces a white streak and an intense blue-white fluorescence under shortwave UV that is nearly diagnostic for the species. Using both tests together can resolve ambiguous identifications where one test alone is insufficient, particularly within the carbonate and tungstate mineral groups.
What is the streak of gold and silver, and does it help identification?
Gold produces a golden-yellow streak, and silver produces a silver-white to light gray streak, both of which match or closely approximate their surface colors. For native elements in general, streak is less diagnostic because the metal pigment is the same regardless of surface oxidation state. The streak test is most useful for gold in distinguishing it from pyrite (fool’s gold): pyrite streaks greenish-black while gold streaks yellow. The malleability test is even more definitive—gold is highly ductile and leaves a bright mark when pressed against a surface, while pyrite is brittle and produces a black powder under the same pressure. For silver, the streak combined with a tarnish pattern and specific gravity (10.5) provides reliable identification from similar-looking argentite (acanthite), which streaks gray-black.
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