Information on jewelry stamps or marks is available from a variety of sources including Jewelers.org and About.com. Amazon.com offers several books with information on jewelry marks such as "Warman's Jewelry: Identification & Price Guide."
Jewelry stamps, also known as hallmarks, on precious metals such as platinum, gold and silver indicate the country of origin, purity of the metal and identification of the manufacturer. Jewelry.com, the website of Jewelers of America, is a good reference for information on precious metal stamps. The website 925-1000.com, an online encyclopedia of silver marks, offers photos and information on silver stamps from all over the world. About.com also provides examples and photos of costume and vintage costume jewelry.