What Is 999 Plus Silver?

Millesimal Fineness

• Silver purity is graded by millesimal fineness using the percentage of the element. For example, a ring made up of 90 percent silver and 10 percent copper would be 900 pure silver. This is the universal standard for grading silver and is the rating most used by dealers and collectors. The millesimal system is based on parts per 1,000 and is an offshoot of the carat concept used for gold and diamonds.

999 Plus Pure Silver

• The highest and most expensive silver is the 999 Plus pure silver. This means it is 99.9 or more percent silver and has little or no impurities. This is most commonly used for bullion bars. Silver of this purity is extremely susceptible to bending and is not traditionally suitable for jewelry. Collectors and sellers call silver of this purity "three nines fine."

Sterling Silver

• The most common grade of silver used in jewelry and other high-end items is about 92.5 percent pure and is mixed with about 8 percent copper or other base metal. It has a fineness of 925 and is the lowest fineness marked as fine silver. Sterling silver is often plated with a thin layer of 999 Plus pure silver to add shine.

Copper Mixing

• 999 pure silver is sometimes mixed with 10 percent copper, which adds strength, but also lowers the quality. Since it is no longer pure, the silver can not be considered 999 Plus pure. Copper does not discolor the silver, so some unscrupulous sellers continue to call it 999 Plus pure despite the copper inclusion.