The enduring allure of gold has led to countless instances of mistaken identity, most famously involving the mineral known as pyrite, or iron sulfide (FeS₂). Pyrite has a distinct brassy-yellow color and a metallic sheen that causes people to confuse it with elemental gold (Au). This common mix-up has earned pyrite the nickname “Fool’s Gold.” The key to definitively telling these two minerals apart lies in examining their physical properties, specifically their resistance to scratching.
Measuring Mineral Hardness
Geologists use a standardized system to measure a mineral’s resistance to being scratched, known as the Mohs Scale of Mineral Hardness. This qualitative scale assigns a number from 1 to 10 based on the ability of a harder material to visibly scratch a softer one. The scale is purely ordinal, meaning a mineral with a higher number is simply harder than any mineral with a lower number.
The Mohs scale utilizes ten reference minerals, ranging from talc at a hardness of 1 to diamond at 10, the hardest known naturally occurring substance. Common objects also fall onto this scale, providing practical benchmarks for field testing. For instance, a copper penny generally has a hardness of about 3.2, and a steel knife blade or glass plate registers around 5.5 to 6.5. This comparative method allows for a quick and simple assessment of scratch resistance.
The Hardness Verdict: Pyrite vs. Gold
When comparing the two minerals, the difference in scratch resistance is immediate and substantial. Pure gold is a relatively soft metal, registering a low hardness value of 2.5 to 3 on the Mohs scale. This means that gold is easily scratched by a copper coin and can even be scratched by an ordinary fingernail in some cases.
Pyrite, the iron sulfide mineral, is significantly harder, possessing a Mohs hardness that ranges from 6 to 6.5. This greater hardness allows pyrite to scratch glass and resist scratching from a steel knife blade. The practical implication is that if a specimen can be easily scratched or deformed by a simple metal tool, it is likely gold, but if it resists scratching and can scratch glass, it is pyrite.
The vast difference in hardness stems from their distinct chemical structures. Gold exists as a native element with metallic bonding, which allows the atoms to slide past one another easily, resulting in its characteristic malleability and softness. Pyrite, an iron sulfide compound, has a strong crystalline structure, making it much more resistant to scratching and deformation. Pyrite is much harder than gold.
Beyond Hardness: Distinguishing Properties
While hardness is a definitive test, other unique physical properties provide further confirmation to distinguish gold from pyrite.
Streak Test
One of the most reliable tests is the streak test, which involves rubbing the mineral across an unglazed porcelain plate to observe the color of its powdered form. Gold leaves a distinct yellow or golden streak. Pyrite, composed of iron and sulfur, produces a greenish-black to brownish-black streak when tested. This difference is a crucial identifier because a mineral’s streak color is often more consistent than its external color.
Density (Specific Gravity)
Another major difference is the specific gravity, a measure of density compared to water. Gold is an exceptionally dense element, with a specific gravity of approximately 19.3, meaning a piece of gold is about 19.3 times heavier than an equal volume of water. Pyrite is also heavy but significantly less dense, with a specific gravity ranging from 4.9 to 5.2. A nugget of gold will feel surprisingly heavy for its size, while a piece of pyrite of similar size will feel substantially lighter.
Crystal Structure and Tenacity
The crystal structure and tenacity offer visual and mechanical clues. Gold is highly malleable and ductile, meaning it can be hammered flat or bent without breaking; natural gold often forms irregular nuggets, flakes, or wires. Pyrite, by contrast, is brittle and tends to shatter or crumble when struck. Pyrite characteristically forms distinct, well-defined cubic or octahedral crystals, often displaying fine parallel lines called striations on its faces.