The visual similarity between gold and the iron sulfide mineral known as pyrite, or “Fool’s Gold,” has historically led to countless misidentifications. Pyrite’s metallic sheen and pale yellow color can deceive the untrained eye, making it appear valuable when it is essentially worthless. Accurate identification is necessary for prospectors, geologists, and collectors to distinguish the precious elemental metal from the common sulfide compound. Several simple yet effective physical tests reveal the fundamental differences between these two minerals, moving beyond mere visual inspection.
Differences in Appearance and Structure
Distinguishing the two minerals involves closely observing their appearance, focusing on subtle differences in color and luster. Gold exhibits a soft, rich, buttery-yellow hue that remains consistent under varying light conditions. Pyrite typically displays a paler, brassy-yellow color that can sometimes appear slightly greenish. Pyrite’s luster is often more intensely metallic and mirror-like, while gold has a softer, more consistent metallic shine.
A major structural difference lies in their crystal habit, or the typical shape they form in nature. Pyrite often forms distinct, geometric shapes, most commonly cubes, octahedrons, or multi-faceted crystals with sharp edges. These crystals frequently show fine, parallel lines, or striations, on their faces. Gold usually occurs as irregular, amorphous nuggets, flakes, or grains, lacking the sharp, defined edges.
Practical Tests of Hardness and Malleability
The Mohs scale of hardness provides a clear separation between the two minerals. Gold is a very soft metal, ranking between 2.5 and 3.0 on the Mohs scale. This low hardness means gold can be easily scratched or dented with a standard copper penny or a pocket knife.
Pyrite is significantly harder, possessing a Mohs hardness of 6.0 to 6.5. Attempting to scratch pyrite with a knife or copper coin will prove unsuccessful, as the pyrite resists the softer material. This hardness difference allows pyrite to scratch glass, a feat impossible for gold.
The reaction of the minerals to blunt force, known as tenacity, further separates them through a test of malleability. Gold is highly malleable, meaning it can be flattened or bent without breaking when struck. It will deform or dent, but not shatter. Pyrite is brittle. When struck with force, pyrite will shatter, crumble, or break into numerous small, sharp fragments rather than flattening.
Definitive Density and Streak Identification
The most reliable methods for positive identification involve the streak test and a comparison of density. The streak test determines the color of a mineral when it is powdered by rubbing a sample across an unglazed ceramic plate. Gold leaves a distinctly yellow or gold-colored streak. Pyrite, when powdered, leaves a dark streak, typically a greenish-black or brownish-black color. This test is often conclusive because the streak color reveals the mineral’s true, internal character regardless of surface appearance.
A comparison of density provides the other definitive physical test, revealing the difference in their structure. Gold is an extremely dense element, with a specific gravity of approximately 19.3. This means gold is over 19 times denser than an equal volume of water, making even small pieces feel unusually heavy. Pyrite has a specific gravity of around 5.0, making it much less dense than gold. A piece of pyrite the same size as a gold nugget will feel noticeably lighter in the hand. Geologists can perform a more precise measurement using a water displacement test, which accurately calculates density.