Understanding Purity
Purity tells you how much of an alloy is precious metal. The marking may appear as a decimal, a percentage, or—especially for gold—as a karat number.
In This Guide
Fineness and Percentage
Fineness expresses precious-metal purity in parts per thousand. A product marked .999 fine contains 999 parts precious metal out of 1,000, or 99.9%. A .900 gold coin contains 90% gold and 10% other metals by weight.
Understanding Gold Karats
The karat system divides gold purity into 24 parts. Twenty-four-karat gold is considered essentially pure; 18-karat gold is 18 parts gold out of 24, or 75%; and 14-karat gold is 14 parts out of 24, or approximately 58.3%.
Karat is a purity measurement and should not be confused with carat, a weight measurement used for gemstones.
Why Metals Are Alloyed
Pure gold and silver are relatively soft. Copper, silver, zinc, and other metals may be added to improve hardness, wear resistance, color, or manufacturing performance.
An alloyed bullion coin can still contain a full stated troy ounce of pure metal. Its gross weight will simply be greater because the alloying metal is additional weight.
Common Purity Markings
| Marking | Meaning | Common Context |
|---|---|---|
| .9999 | 99.99% pure | Many modern gold bullion products |
| .999 | 99.9% pure | Silver rounds and bars |
| .925 | 92.5% silver | Sterling silver |
| .900 | 90% precious metal | Many historic circulating coins |
| 18K or 750 | 75% gold | Gold jewelry |
| 14K or 585 | About 58.5% gold | Gold jewelry |
Calculating Fine-Metal Content
Multiply the item’s total weight by its purity expressed as a decimal. A 10-troy-ounce bar that is .999 fine contains approximately 9.99 troy ounces of pure silver.
For many widely traded bullion products, the mint states the guaranteed fine-metal content directly. Use that published specification rather than trying to infer it only from gross weight.
Testing and Trust
A stamped purity mark is useful, but a mark can be copied. Buy from reputable sources and use appropriate verification for unfamiliar or high-value items. Weight, dimensions, magnetic behavior, electrical testing, specific gravity, and professional analysis can provide different levels of evidence.
- Do not rely on color alone to judge purity.
- Do not assume “24K” automatically proves authenticity.
- Do not damage a valuable coin with an inappropriate scratch or acid test.
- Do not confuse plated material with solid precious metal.
Frequently Asked Questions
Is .9999 gold meaningfully more valuable than .999 gold?
It contains slightly more gold at the same gross weight, but product size, premium, recognition, and guaranteed fine-gold content may matter more in an ordinary purchase.
Can a 22-karat coin contain one full ounce of gold?
Yes. Its total weight can be greater than one troy ounce so that the pure-gold portion still equals one full troy ounce.
What does sterling silver mean?
Sterling silver is an alloy containing 92.5% silver, commonly marked .925.
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Purity is only half of the metal-content equation. Combine fineness with accurate weight to know what you actually own.