Gold, represented by the chemical symbol Au, is a highly valued transition metal prized for its resistance to corrosion and chemical non-reactivity. Its desirability has historically led to the creation of imitations, including plated items or alloys containing very little gold, making authentication a necessary step. Determining the true purity, or karat, of an item requires a methodical approach that moves beyond simple visual assessment. Since counterfeit items often mimic the weight and appearance of genuine gold, verification relies on testing the unique physical and chemical properties inherent to the metal itself. The process of confirming authenticity should progress from simple, non-invasive observations to more complex, scientifically grounded measurements.
Non-Destructive Home Checks
The first step in verification involves a careful visual inspection of the item, specifically searching for stamped markings known as hallmarks. Genuine gold jewelry is typically stamped with a number indicating its purity, such as 750 for 18-karat gold (75% pure) or 585 for 14-karat gold (58.5% pure). However, the presence of certain letters alongside the karat number can suggest the item is not solid gold; for instance, markings like GP (Gold Plated), GF (Gold Filled), or HGE (Heavy Gold Electroplate) mean a thin layer of gold is bonded to a base metal. These markings are not always reliable indicators, as they can easily be faked or stamped onto non-gold items.
A simple yet effective physical test involves observing the metal’s reaction to a common household magnet. Gold is a non-ferromagnetic metal, meaning it will not be attracted to a standard magnet. If the item strongly adheres to the magnet, it likely contains a significant proportion of ferrous metals, such as iron or nickel, and is therefore not solid gold. This test is most effective when the attraction is strong, though small components like clasps or springs may contain steel parts even on authentic pieces.
Another accessible test uses an unglazed ceramic plate or tile to check the metal’s streak, which is the color of the powdered mineral. By lightly dragging the gold item across the rough, unglazed surface, a streak of powder will be left behind. If the streak is bright yellow or gold, the item is likely genuine. Conversely, if the streak is black, dark gray, or greenish-black, the item is more likely to be an iron sulfide mineral like pyrite, commonly known as fool’s gold.
Measuring Density: The Specific Gravity Test
The most definitive physical method for testing gold involves measuring its specific gravity, a property that compares the density of the object to the density of water. Pure gold (24-karat) has a specific gravity of approximately 19.3, meaning it is 19.3 times denser than water. This high density is a unique physical characteristic. The specific gravity value changes predictably with the purity level; 18-karat gold typically measures around 15.4 and 14-karat gold around 13.7, due to the lighter alloy metals mixed in.
To perform this test, the item must first be weighed precisely in air using a sensitive scale. Next, the item is fully submerged in water, often while suspended from the scale by a thin string, to measure its weight while displaced in the liquid. According to Archimedes’ Principle, the difference between the weight in air and the weight in water equals the weight of the water displaced, which allows for the calculation of the item’s volume. Specific gravity is then calculated by dividing the weight in air by the weight lost when submerged.
Any significant deviation from the expected specific gravity for a given karat level suggests the presence of lower-density metals or a hollow core. For example, if an item stamped as 18-karat gold yields a specific gravity closer to 10.5 (the value for pure silver), it indicates a high concentration of a cheaper alloy or a non-gold core. This density measurement is a non-destructive verification method that provides a reliable, scientifically repeatable result for assessing purity.
Chemical Verification Methods
Chemical verification uses the non-reactive nature of gold to determine its purity, often involving specialized acid solutions. The traditional method uses nitric acid, which will dissolve most base metals and alloys commonly mixed with gold, such as copper, zinc, and silver. A small amount of the gold item is scratched onto a touchstone, and a drop of nitric acid is applied to the resulting metallic streak. If the streak dissolves or changes color, it indicates the presence of reactive non-gold metals.
The process employs different acid concentrations calibrated to specific karat levels, such as 10K, 14K, or 18K. If an acid meant for 14-karat gold is applied and the streak remains unaffected, the metal is at least 14-karat purity or higher. Pure gold (24-karat) is inert to nitric acid and will show no reaction, while lower-karat gold will exhibit a mild fizzing due to the dissolution of the alloyed metals. These testing solutions are corrosive and require the use of proper protective equipment, including gloves and ventilation, making this method often best left to experienced professionals.
For the most accurate and completely non-destructive analysis, professional jewelers and assayers use X-Ray Fluorescence (XRF) technology. This advanced method directs X-rays at the metal surface, causing the atoms to emit characteristic secondary X-rays that identify the exact elemental composition and purity. XRF testing provides an immediate, precise measurement of the gold content without requiring any physical modification or chemical damage to the item.