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Sterling silver: 92.5 percent silver, 7.5 percent copper.
The short version. Sterling silver is a legal alloy. The number 925 refers to the parts per thousand of pure silver in the metal. The remaining 75 parts are usually copper, which gives the alloy the strength pure silver lacks. Copper is also the reason sterling tarnishes.
Why the alloy exists.
Pure silver (999 fine) is too soft to hold a ring shank or a clasp. It bends against a keyring inside an hour. Adding copper hardens the metal enough that a signet ring keeps its edge, a chain link resists deformation, and a bezel setting holds a stone. The 92.5 / 7.5 ratio is the compromise between hardness and colour: below 90 percent silver the alloy starts to look yellow, above 95 percent it goes back to being too soft.
Why it tarnishes.
Copper reacts with sulphur compounds in air, sweat, and water to form silver sulphide, which is dark and dull. The rate depends on humidity, skin chemistry, and whether the piece is worn or stored in a sealed pouch. On daily-wear sterling, expect a first visible tarnish tint within one to three months. Full explainer and cleaning method on the tarnish page.
Alternatives you may see.
- Argentium (935 or 960): silver alloyed with germanium instead of copper. Tarnishes far less; priced 15 to 25 percent above 925.
- Fine silver (999): nearly pure. Very soft; usable only in low-stress pieces like earrings.
- Coin silver (900): lower silver content, harder, historically used for cutlery. Not standard in demi-fine.
- Silver-plated: a base metal with a thin silver layer. Not sterling. Wears through within a year on high-contact pieces. See silver-plated tarnish.
Buying sterling silver.
- Check for the 925 stamp inside the shank or on the clasp tag. Any reputable house hallmarks.
- Avoid pieces sold as “silver-tone” or “silver-plated over brass”. These are plated, not sterling.
- Rhodium-plated sterling looks whiter and resists tarnish for as long as the rhodium layer holds (usually 12 to 18 months on daily wear).