Understanding Gold Purity: 22ct vs 24ct

The carat (or karat in American English) measures the proportion of pure gold in an alloy. Pure gold is 24 carats. A 22-carat coin contains 91.67% gold by weight, with the remaining 8.33% being other metals — typically copper, silver, or a combination of both.

The carat system dates back centuries and is one of the oldest standards in metallurgy. Each carat represents 1/24th of the total composition, so 22-carat gold is 22 parts gold and 2 parts other metals.

Coins minted from 99.99% pure gold (often marked as .9999 fine) include the Canadian Maple Leaf, Austrian Vienna Philharmonic, Australian Kangaroo, and Chinese Panda. These coins contain virtually no alloying metals.

Pure gold is softer than alloyed gold, making 24ct coins more susceptible to scratches and dings from handling. Many modern 24ct coins include anti-counterfeiting features like the Maple Leaf’s micro-engraved privy mark or the Philharmonic’s diffractive security feature.

From an authentication perspective, 24ct coins have a conductivity of approximately 44.0 MS/m and a density of 19.30 g/cm³. These values are consistent across all 24ct coins regardless of the mint, because the composition is essentially identical.

The most famous 22ct coins are the South African Krugerrand (91.67% gold, 8.33% copper), the British Sovereign (91.67% gold, 8.33% copper), and the American Gold Eagle (91.67% gold, 3% silver, 5.33% copper). Each has a slightly different alloy recipe.

The copper content gives 22ct coins a distinctive warmer, more reddish hue compared to the bright yellow of pure gold. It also makes them harder and more resistant to wear, which is why historically circulating gold coins were almost always alloyed.

Because the alloy composition varies between coin types, each has a unique conductivity. A Krugerrand (Au-Cu alloy) has a conductivity of approximately 9.7 MS/m, while an American Gold Eagle (Au-Ag-Cu alloy) measures approximately 8.5 MS/m. This difference is significant for authentication and means each coin type needs its own reference value.

Gold’s electrical conductivity drops dramatically when alloyed. Pure gold conducts electricity at 44.0 MS/m, but adding just 8.33% copper reduces this to approximately 9.7 MS/m — a four-fold reduction. This phenomenon, known as the Nordheim effect, occurs because the foreign atoms disrupt the crystal lattice and scatter conduction electrons.

For eddy current testing, this means 22ct coins produce much less damping than 24ct coins of the same size. A 1 oz Krugerrand (22ct) gives a pendulum reading of approximately 42 degrees, while a 1 oz Maple Leaf (24ct) gives approximately 6 degrees. Both readings are correct for their respective compositions.

The EON reference database includes predicted readings for every major coin type, accounting for the specific conductivity of each alloy. This is why selecting the correct coin type before testing is essential for accurate authentication.

Both 22ct and 24ct coins contain the same amount of gold when measured in troy ounces. A 1 oz Krugerrand and a 1 oz Maple Leaf both contain exactly 1 troy ounce (31.1035 g) of pure gold. The Krugerrand simply weighs more overall (33.93 g) because of the additional copper.

Premiums over spot price vary by coin type, mint, condition, and market demand rather than by purity alone. Some investors prefer 24ct coins for their simplicity, while others choose 22ct coins for their durability and historical significance. Both are equally valid as gold bullion investments.

When buying or selling, the key is to know exactly what you have. Understanding whether your coin is 22ct or 24ct, and which specific type it is, ensures you price it correctly and authenticate it against the right reference values.