The search for gold often leads to confusion with a naturally occurring mineral called pyrite, commonly known as fool’s gold. Pyrite is an iron sulfide, while real gold is an elemental metal. Both substances share a deceptive metallic luster and a yellowish hue, which frequently misleads prospectors and rock collectors. Understanding the distinct physical and chemical properties of each material is necessary for accurate identification. This differentiation process begins with a simple visual inspection before moving to more scientific tests.
Visual Identification: Color, Luster, and Crystal Shape
Real gold displays a distinctive, rich, buttery yellow color that remains consistent across the sample. Pyrite, in contrast, often presents a paler, brassier yellow hue, sometimes with a slight greenish or tarnished tint. While both exhibit a metallic sheen, pyrite’s luster is frequently described as brighter and more brilliant, almost like highly polished metal. Gold typically possesses a softer, deeper metallic gleam.
The structure of the material provides another immediate visual clue. Gold rarely forms perfect, geometric shapes in nature, typically appearing as irregular flakes, amorphous nuggets, or thin wires. This lack of defined structure is characteristic of the native element.
Pyrite, however, often crystallizes into distinct, well-formed geometric shapes. These pyrite crystals commonly take the form of perfect cubes, octahedrons, or pyritohedrons. The presence of these sharp, repeated, and highly structured crystal faces is a strong indicator that the sample is fool’s gold. If the piece is merely an irregularly shaped mass, further testing is required.
Testing the Weight: Specific Gravity and Density
Moving beyond visual cues, testing the material’s density provides a quick and reliable means of differentiation. Density is measured by specific gravity, which compares the mass of a substance to the mass of an equal volume of water. Gold is an exceptionally dense metal, possessing a specific gravity of approximately 19.3. This means a piece of gold is over 19 times heavier than the same volume of water.
Pyrite is significantly less dense than gold, with a specific gravity of around 5.0. This difference allows for a simple “heft” test, where the weight of the material is assessed relative to its size. If a sample feels surprisingly light for its volume, it is likely iron sulfide.
A sample that feels exceptionally heavy in the hand is a strong positive indicator for real gold. This substantial difference in mass, almost a four-fold increase over pyrite, is one of the most defining physical characteristics of elemental gold. This simple test can often eliminate pyrite before more destructive measures are required.
The Definitive Tests: Hardness and Streak
The most conclusive tests involve physical manipulation, starting with an assessment of the material’s hardness and reaction to pressure. Gold is a relatively soft metal, ranking between 2.5 and 3 on the Mohs scale. A piece of pure gold is highly malleable, meaning it can be easily bent, dented, or flattened without breaking when struck or pressed firmly.
Pyrite, by contrast, is much harder, ranking between 6 and 6.5 on the Mohs scale, and cannot be scratched by common objects like a copper penny or a steel knife. Pyrite is brittle; when struck or placed under pressure, it will shatter, crumble, or fracture into smaller pieces. The observation that a sample bends rather than breaks is a definitive marker for elemental gold.
The final and most reliable method for distinguishing the two is the streak test, which examines the color of the mineral when powdered. This test requires rubbing the sample across an unglazed porcelain plate, commonly called a streak plate. The resulting powder color removes any ambiguity caused by surface tarnish or visual deception.
Real gold leaves a metallic yellow streak that closely matches the color of the nugget itself. This consistency is due to the sample being an element and not a compound. Pyrite, however, leaves a distinctly dark streak, typically appearing as a greenish-black or brownish-black powder. This stark difference provides the clearest evidence that the substance is iron sulfide and not gold.