The streak test is a simple, reliable method used by geologists and prospectors to help identify unknown minerals. This process involves scraping a mineral sample across a rough, unglazed surface to produce a line of its powdered form, known as its streak. While the outward appearance of a mineral, such as its color or luster, can be misleading due to impurities or surface tarnish, the color of its powder is generally consistent and unchanging. The streak color therefore provides a true, diagnostic characteristic.
The Direct Answer Gold’s Streak Color
Real gold leaves a streak that is characteristically a bright, metallic yellow or gold color. When pure gold is dragged across an abrasive surface like sandpaper, the resulting line of powder retains the metal’s true color. This distinct result is related to gold’s low position on the Mohs hardness scale, where it ranks between 2.5 and 3. This softness means the material is scraped off and smeared onto the abrasive surface rather than being pulverized into dust. This physical property is a quick indicator of authenticity, as few other minerals that resemble gold share this soft, yellow-streaking characteristic.
The Mechanism of the Streak Test
The streak test relies on the difference in hardness between the mineral and the abrasive surface, grinding the mineral into a fine powder whose color can then be observed. For consistency in geology, the standard tool is an unglazed porcelain streak plate, which has a consistent Mohs hardness of about 6.5 to 7.
Sandpaper functions similarly, using an abrasive surface that is harder than the sample being tested. The sandpaper’s grit acts to scratch the mineral, physically removing a layer of the sample. This powder color is a more reliable identification feature than the mineral’s surface color, which can be altered by weathering or tarnish. For minerals softer than the abrasive surface, the test successfully produces a streak of powder, while harder minerals will simply scratch the plate or paper and not leave a powder streak.
Distinguishing Gold from Common Look-Alikes
The consistency of the streak test makes it highly effective for differentiating real gold from minerals often mistaken for it. The most common “fool’s gold” is Pyrite, which has a brassy, metallic yellow appearance that closely mimics true gold. When Pyrite is subjected to the streak test, however, it leaves a distinct black or brownish-black streak.
Another common look-alike is Chalcopyrite, which also has a brassy to golden-yellow color. Unlike real gold, Chalcopyrite is a copper iron sulfide mineral, and its powdered form yields a greenish-black streak.