925 Sterling Silver: What the Mark Means, Tarnish and Care

A 925 stamp means the metal is 925 parts per thousand pure silver, or 92.5 percent, which is the standard for sterling. The remaining 7.5 percent is traditionally copper. That copper is also the main reason sterling tarnishes faster than pure silver, so storage matters as much as polishing.

What 925 means

Sterling silver is an alloy of at least 92.5 percent silver. The Canadian Conservation Institute describes it as 92.5 percent silver and 7.5 percent copper. The number on the piece is its fineness, the parts per thousand of pure silver, and it may appear as “925,” “Sterling” or “Ster.”

Other silver standards in common use:

Name	Fineness (parts per thousand)	Status
Fine silver	999	UK hallmarking standard
Britannia silver	958	UK hallmarking standard
Sterling silver	925	US minimum for “sterling”; UK hallmarking standard
Coin silver	900	US minimum for “coin silver”
800 silver	800	Lowest UK hallmarking standard; may be sold in the US as “800 silver”

The UK figures come from the Hallmarking Guidance Notes of the UK assay offices, which list 999, 958, 925 and 800 as the legal silver standards. The same document lists optional traditional symbols for sterling and Britannia silver that can be struck beside the hallmark. How those marks fit together is explained in Jewelry Hallmarks and Quality Marks. GIA’s ring material comparison rates sterling rings as resizable by up to about two sizes when no gems are set; sizing itself is covered in the ring size chart.

US rules in plain English

The FTC Jewelry Guides (16 CFR 23.5) and the National Gold and Silver Stamping Act cover silver. This is a summary, not legal advice.

• “Sterling,” “Sterling Silver,” “Ster.” and “solid silver” should be used only for metal that is at least 925 parts per thousand pure silver.
• “Coin” and “coin silver” require at least 900 parts per thousand.
• “Silver” on its own should be used only with an equally prominent fineness number immediately before it, such as “800 silver.” Since the 2018 revisions, that allows alloys below 925 to be called silver if the fineness is stated, while the 925 and 900 thresholds for “sterling” and “coin” remain.
• Plated pieces may be described as silver only if the description makes clear the item is only coated, and the coating must cover all significant surfaces and be reasonably durable. Under 15 U.S.C. 297, the words “sterling” and “coin” may not be stamped on plated articles at all.
• Trademark. The same section requires a manufacturer or importer that applies a silver quality mark to add its registered trademark or name next to it, in lettering at least as large.
• Rhodium. 16 CFR 23.7 says a rhodium surface layer on products described as precious metal should be disclosed. The 2018 notice gives rhodium-plated silver as an example. Platinum’s own marking rules, which sit in the neighboring section, are summarized in Platinum in Jewelry.

Tolerances

Under 15 U.S.C. 296, a silver part tested without solder may fall short of its mark, including the sterling and coin standards, by no more than four parts per thousand. When the whole article is assayed with its solder, the allowance is ten parts per thousand.

Why silver tarnishes

Tarnish on silver is silver sulfide (Ag₂S, the mineral acanthite), which forms when silver reacts with sulfur-containing gases in the air. The most common is hydrogen sulfide, the gas given off by a hard-boiled egg. According to the Canadian Conservation Institute:

• Color shows thickness. As the film grows from about 10 to 100 nanometers thick, it passes from yellow through red-brown to blue. Thicker films are black, the true color of silver sulfide.
• Sterling tarnishes faster than pure silver, because copper is more reactive than silver.
• Humidity speeds it up. Silver tarnishes faster as relative humidity rises, and CCI recommends storing it below 50 percent RH.
• Household sources of sulfur gases include vulcanized rubber (rubber bands, latex and rubber gloves, pencil erasers), wool and wool felt, and certain paints.
• Every polish costs metal. Each time silver tarnishes and is cleaned, some silver is lost. On silver-plated items, polishing can thin or remove the plating.

Tarnish-resistant alloys

Some modern sterling alloys are made to slow tarnish. The best known, Argentium, is at least 92.5 percent silver, like ordinary sterling, but replaces some of the copper with a small amount of germanium. Cynthia Eid’s account on Ganoksin describes it as highly tarnish resistant rather than tarnish-proof, and notes that it does not form firescale when heated. Alloys like this still qualify as sterling, so a 925 stamp alone does not say which sterling alloy is in the piece.

Storage

CCI’s metal care guidelines are written for museum collections, but they translate well to a jewelry box:

1. Store silver in sealed enclosures. Polyethylene zip bags with the air squeezed out work; clear polyester (PET) bags give better protection against sulfur gases.
2. Wrap pieces in acid-free tissue before bagging, since slip agents inside some plastic bags can scratch silver.
3. Add something that captures sulfur, such as a tarnish-preventing cloth, and keep the enclosure dry with a desiccant.
4. Keep silver away from rubber bands, wool felt and freshly painted or sealed wood.
5. Handle polished pieces with clean nitrile gloves, as CCI recommends for polished silver.

CCI notes that if tarnish keeps coming back quickly, the storage needs improving. More polishing is not the answer.

Cleaning

For light tarnish on sterling jewelry, GIA recommends a cleaner made specifically for silver jewelry or a silver polishing cloth. Because every polish removes a little metal, light and occasional cleaning is better than frequent heavy polishing.

A few practical points:

• Check for stones first. Pearls, turquoise, coral and other porous or organic gems set in silver should be kept away from silver dips and chemicals. Gem-by-gem guidance is in How to Clean Jewelry Safely.
• Treat plated silver gently. Rhodium-plated and gold-plated silver should be wiped with a soft cloth and mild soapy water, not rubbed with abrasive polish, which can wear through the plating. Gold over sterling is sold as vermeil when it meets the FTC’s thickness and karat standards, explained in Gold Plated vs Gold Filled vs Vermeil.
• Dry pieces fully after any wet cleaning before they go back into storage, since damp conditions speed tarnish.

Common questions

Is 925 silver real silver? Yes. It is sterling silver, 92.5 percent pure silver. The term “925” and the word “sterling” mean the same standard.

Does a 925 stamp prove a piece is sterling? No. In the US the maker or importer applies the mark, which is why the law requires a trademark or name beside it. In the UK, silver articles of 7.78 grams or more must carry a full hallmark from an assay office before they can be described as silver, which is independent confirmation.

Is “silver plated” the same as sterling? No. A plated piece has a thin silver layer over another metal. In the US it may not be stamped “sterling,” and it should be described as plated.