Finding a bright, metallic yellow specimen in the field can be an exhilarating moment. This excitement must be tempered with verification, as several common minerals closely mimic the appearance of gold. Simple field tests are an important step before celebrating a potential treasure. A systematic approach using physical and chemical properties can quickly determine whether a find is truly the precious metal or merely a shiny imitation.
Initial Visual Cues and Common Look-Alikes
Real gold exhibits a rich, buttery yellow color and maintains a bright, consistent metallic luster, even in dim light. The metal does not tarnish. In nature, gold is usually found as irregular, rounded nuggets, flakes, or amorphous masses, often water-worn with soft edges.
The most frequent impostor, pyrite, often called “Fool’s Gold,” displays a paler, brassy-yellow hue and a flashier, glittery metallic sheen. Pyrite is an iron sulfide mineral that can sometimes develop an iridescent tarnish due to oxidation. Pyrite often forms well-defined, geometric cubic or octahedral crystals with sharp edges, a shape rarely seen in natural gold. Another common mimic is mica, which appears as glittering, flaky sheets that are much softer and lighter.
Simple Field Tests: Weight and Malleability
The difference in density between gold and its mimics is one of the most immediate and reliable field indicators. Gold is an extremely dense element, possessing a specific gravity of approximately 19.3. This means a piece of gold will feel surprisingly heavy compared to a similar-sized piece of rock or mineral. Pyrite, by contrast, has a significantly lower specific gravity of about 5.0, making it feel substantially lighter for its volume.
The malleability of gold, its ability to be deformed without breaking, provides another simple, non-destructive test. Gold is soft, ranking between 2.5 and 3 on the Mohs hardness scale, and can be easily bent or dented under pressure. If a sharp tool like a knife or pin is pressed against the specimen, real gold will yield and flatten without fracturing. Pyrite, being much harder and brittle, with a Mohs hardness of 6 to 6.5, will resist the pressure and instead crack, shatter, or crumble into powder. Pure gold is also non-magnetic, a property that can quickly rule out some iron-bearing minerals.
The Definitive Scratch Test
The streak test determines the color of a mineral in its powdered form and is a definitive field method for identification. This test requires an unglazed ceramic plate, known as a streak plate. The specimen is firmly scraped across the rough surface of the plate, leaving a small line of powdered material behind.
If the specimen is true gold, the resulting streak will be a continuous, bright yellow, mirroring the color of the metal itself. If the specimen is pyrite, the resulting streak will be a greenish-black or brownish-black color. This visible contrast between the powder colors of gold and its most common imposter is a reliable diagnostic tool.
Final Verification Methods
For conclusive verification beyond visual and physical field tests, further methods involving chemical properties and professional analysis are necessary. The most common chemical test capitalizes on gold’s status as a noble metal, meaning it resists corrosion and is chemically inert. Nitric acid is frequently used because it will dissolve base metals and other metallic sulfides, such as pyrite, but will not react with gold.
To perform this, a small mark of the specimen is made on a specialized black testing stone, and a drop of nitric acid is applied to the streak. If the streak is not gold, it will effervesce or dissolve away, confirming it is a base metal or sulfide. Because handling corrosive chemicals like nitric acid requires careful safety protocols, it is prudent to leave such chemical testing to professionals or use commercially prepared, low-concentration testing kits. For the most accurate and certified result, the final step involves submitting the sample to a professional assay laboratory or a qualified geologist.