Those little decimal numbers on every bar and coin are a complete language. It takes two minutes to learn.
The tiny decimal stamped on every bar and coin is a complete language with a thousand-year grammar. Two minutes here and you will read it fluently for life.
Purity, or fineness, is expressed in parts per thousand of pure metal. A Morgan dollar is .900 fine: 900 parts silver, 100 parts copper. An American Silver Eagle is .999. A Canadian Maple Leaf is .9999, four nines. Multiply an item’s gross weight by its fineness and you have its pure metal content, the only two numbers bullion arithmetic ever needs.
Circulating coinage was deliberately alloyed because pure gold and silver are soft. The copper in a .900 dollar is what let it survive decades of pockets, purses, and cash drawers with its design intact. The British perfected crown gold, 22 karat, .9167 fine, for exactly this reason, and it lives on today in the Krugerrand and the American Gold Eagle: coins whose alloy trades a little purity for real-world toughness.
Jewelry speaks in karats, twenty-fourths of pure gold: 24k is pure, 22k is .9167, 18k is .750, 14k is .583. Fineness and karat are two dialects of the same statement, and converting between them is just division by 24. Sterling silver’s .925 and old U.S. coin silver’s .900 are the same idea in silver’s wardrobe.
Between .999 and .9999, the additional pure metal in a one-ounce coin is measured in cents; the pricing differences you see owe far more to brand, design, and demand than to the fourth nine. What the extra nines really certify is refining technique, modern electrolytic refining reaches purities the ancients could not imagine, verified by the assay traditions that police the whole system.