Accurately identifying gold is often a necessary first step, whether for appraising an inherited piece of jewelry, validating a potential investment, or prospecting for raw material. Gold is a noble metal, meaning it resists corrosion and oxidation, which makes it highly valued and durable. Distinguishing genuine gold from common imitations or alloys relies on understanding its unique physical and chemical properties. The goal of using various tests is to move from simple visual checks to more scientific methods to confirm the purity and authenticity of the metal.
Initial Visual Identification
The first step in assessing a gold item involves a careful, non-destructive visual examination of the metal’s surface. A strong indicator of authenticity is the presence of a hallmark, a small stamp or inscription pressed into the metal. These marks denote the item’s purity, typically expressed in karats (K) or by a millesimal fineness number. For instance, “14K” indicates the item is 58.3% pure, while a millesimal mark of “750” signifies 75.0% pure gold, equivalent to 18K.
The absence of a purity stamp does not automatically mean the item is fake, especially with older jewelry, but its presence is a strong initial sign of value. Conversely, stamps including “GP” (Gold Plated), “GF” (Gold Filled), or “HGE” (Heavy Gold Electroplate) indicate a microscopic layer of gold over a base metal. The item’s overall color consistency should also be scrutinized, particularly in high-wear areas like clasps, edges, and ring shanks. Plating is extremely thin, often between 0.5 and 2.5 microns, and will eventually wear away to expose the underlying metal, which may appear silvery, gray, or coppery. Solid gold, by contrast, maintains its color throughout, even if scratched.
Simple Physical Property Tests
Gold possesses physical characteristics that are easily tested using common household items, primarily focusing on its density and non-magnetic nature. Pure gold is diamagnetic, meaning it is not attracted to a magnet. A simple test involves holding a strong magnet, such as a rare-earth magnet, near the item. If the item is strongly attracted, it is likely a base metal like steel or nickel, often used in alloys or as a core for plating.
A more reliable physical test is based on gold’s extreme density. The pure metal has a specific gravity of 19.3, meaning it is 19.3 times denser than water, making it one of the heaviest naturally occurring elements. A genuine gold object will feel surprisingly heavy for its size compared to common metals like brass or copper. This density advantage is the basis for a non-destructive specific gravity test, which requires a precise jeweler’s scale and a container of water.
To perform this measurement, the item is first weighed in air, and then re-weighed while fully submerged in water, ensuring it does not touch the container’s sides or bottom. The calculation involves dividing the weight in air by the difference between the weight in air and the weight in water. This final number should closely match the known specific gravity for the suspected karat: 19.3 for 24K, or slightly lower for common alloys like 18K (15.2 to 17.6) or 14K (12.9 to 14.6).
Advanced Testing Methods
The most definitive and widely used professional method is the scratch test, which utilizes a nitric acid kit. This method requires rubbing the gold item firmly onto a touchstone, a slab of unglazed black ceramic, to leave a visible streak of metal. A series of nitric acid solutions, each formulated to test a specific karat purity, is then applied to the streak to observe the chemical reaction. Safety precautions, including wearing gloves and ensuring good ventilation, are necessary due to the corrosive nature of the acids.
The test exploits the fact that gold is highly unreactive, unlike the base metals alloyed with it, such as copper, silver, and zinc. If a streak is exposed to 14K test acid and completely dissolves, the gold content is lower than 14K. Conversely, if the streak remains bright and unaffected, the gold content is equal to or higher than 14K. By testing streaks with progressively stronger acids, the user can bracket the purity level, as the acid dissolves the base metals but leaves the gold component intact.
Identifying Raw Gold in Nature
Identifying raw gold, often found as nuggets or flakes (placer gold), requires distinguishing it from common look-alikes like pyrite (“Fool’s Gold”) and mica. Raw gold has a distinct, rich, buttery yellow color, whereas pyrite typically presents with a brassier, pale yellow hue and a more glittery, sharp luster. Pyrite often forms in distinct cubic or angular crystal shapes, while soft gold is usually found in irregular, rounded, or flattened shapes due to erosion.
The most reliable field test for raw gold is checking its malleability and hardness. Pyrite is an iron sulfide with a Mohs hardness of 6 to 6.5, making it brittle; if struck, it will shatter or break into pieces. Gold, however, is extremely soft, with a Mohs hardness of 2.5 to 3; it will easily deform, bend, or flatten without breaking, which is a clear indicator of genuine gold.
The streak test can also be performed with raw gold by rubbing it against an unglazed ceramic surface. True gold leaves a yellow or gold-colored streak, matching the metal itself. Pyrite, due to its iron content, leaves a distinct greenish-black or brownish-black streak, making the distinction immediate and conclusive. Additionally, gold maintains its metallic shine even in shadow or when found in water, unlike mica and other look-alikes that often appear duller.