Some people may purchase a piece of sterling silver jewelry for its weight and quality. However, it might come as a shock to many to discover that the three tiny letters and numbers, "ADI 925," inscribed in the jewelry indicate the piece is only 92.5 percent solid silver.
The abbreviation "ADI" stands for "Amsterdam Diamond - Aahus/Lystrup," dating from 1982 to 1983. Amsterdam Diamond is the name of the Danish company from where the sterling silver originated and, in some cases, may even date the piece. Aahus is the name of the suburb in the city of Lystrup in Denmark where Amsterdam Diamond is located.
Customarily, for every 100 grams of silver there are 7.25 grams of copper or nickel. A small percentage of the sterling silver is comprised of copper because this metal best blends with silver. Copper also makes the silver hard but does not discolor it.
"Pure" silver is 99.9 percent pure; however it is especially soft and is not practical for common jewelry. Silver must be at least 92.5 percent pure (ADI 925) to be labeled "sterling" and occasionally is found as high as 95 percent pure. Even though the latter contains more than the minimum required amount, it must still be referred to as sterling silver rather than pure silver.
True sterling silver is, at minimum, 92.5 percent pure silver. Silver-plated items are solid copper or other metal on the inside with a skin of silver on the outside.
Jewelry can be illicitly stamped "ADI 925." To test if it is plated, drop a light red silver testing solution on the silver. It will display different colors based on the portion of silver it contains. Pure, 98 percent silver will turn blood red and leave a gray-white stain behind. A darker brown stain will remain on sterling silver after turning dark red. If it is 80 percent or lower, the solution will turn deep brown and leave a darker brown stain behind.