To the untrained eye these numbers mean little, but with knowledge of their meanings you can easily identify fine pieces. Numbers are usually found on the posts of earrings, the inside of rings, on the back of pendants, and on the clasps of necklaces or bracelets. The numbers are used to measure the precious metal content of the piece.

Silver:

Fine silver, consisting of 99.9% silver and 1% other metals, is marked "999." Britannia silver, consisting of 95.84% silver and 4.16% other metals (usually copper) is marked "958." Sterling silver, consisting of 92.5% silver and 7.5% other metals (usually copper) is marked "925". Other markings for sterling silver include "STER", "Sterling", and "Sterling Silver". Argentium™ silver, consisting of 92.5% silver and 6.4% other metals (usually copper) and 1.1% germanium is marked " 925. AS", and "AS 925". Some pieces of Argentium silver are marked with the same markings as traditional sterling silver. Argentium silver is primarily identified by it's slower rate of tarnish. Coin silver consisting of 90% silver and 10% other metals is marked "900."

Gold

24 karat gold, consisting of 100% pure gold, is marked 24kt. 23 karat gold, consisting of 95.8% gold and 4.2% other metals, is marked 23kt. 22 karat gold, consisting of 91.6% gold and 8.4% other metals, is marked 22kt. 21 karat gold, consisting of 87.5% gold and 12.5% other metals, is marked 21kt. 20 karat gold, consisting of 83.3% gold and 16.7% other metals, is marked 20kt. 19 karat gold, consisting of 79.1% gold and 21.9% other metals, is marked 19kt. 18 karat gold, consisting of 75% gold and 25% other metals, is marked 18kt. 17 karat gold, consisting of 70.8% gold and 29.2% other metals, is marked 17kt. 16 karat gold, consisting of 66.6% gold and 33.4% other metals, is marked 16kt. 15 karat gold, consisting of 62.5% gold and 37.5% other metals, is marked 15kt. 14 karat gold, consisting of 58% gold and 42% other metals, is marked 14kt. 13 karat gold, consisting of 54.1% gold and 45.9% other metals, is marked 13kt. 12 karat gold, consisting of 50% gold and 50% other metals, is marked 12kt. 11 karat gold, consisting of 45.8% gold and 54.2% other metals, is marked 11kt. 10 karat gold, consisting of 42% gold and 58% other metals, is marked 10kt. 9 karat gold, consisting of 37.5% gold and 62.5% other metals, is marked 9kt.

Platinum

Platinum jewelry consisting of 100% pure platinum is marked "Pt1000". "Pt" is the atomic symbol for platinum. Platinum jewelry consisting of 95% pure platinum, and 5% alloy is marked "Pt950". Platinum jewelry consisting of 90% pure platinum, and 10% alloy is marked "Pt 900". Platinum jewelry consisting of 85% pure platinum, and 15% alloy is marked "Pt850".

Stainless Steel:

High quality stainless steel jewelry is marked "316L". Other qualities of stainless steel are marked as "Stainless" or "Stainless Steel".

Other Common Markings

Although these markings are not numbers they are important to recognize when identifying jewelry crafted from precious metals. Gold filled jewelry is marked "GF" and is usually accompanied by a fraction; for example:1/20 10K. The process used to craft gold filled jewelry is a heavy form of gold plating, which is laid over a base metal (usually brass). Heavy Gold Electroplate is marked "HGE". Other forms of Gold plate are marked "KP" (Karat Plumb, the exact amount of gold within the piece), "K.P." (Karat Plate), and "G.S" (Gold Shell).

Source:

Federal Trade Commission: Facts for Consumers

Pearlmans Jewelers: Platinum 101

Fire Mountain Gems: The Meaning of Medals

Resource:

ModernSilver.com: Basic Hallmark Identification