The quality mark for sterling silver is 925; 825 refers to a grade of continental silver. By law, all silver that is marked with the grade of its purity must also contain the name or registered trademark of the company that certifies this purity.
As pure silver is too soft for most applications, base metals, such as copper, are usually added to increase its hardness and durability. Sterling silver is defined as a mixture containing 92.5 percent, or 925 parts out of 1,000, of pure silver. "Coin silver" is another recognized silver designation that refers to compounds containing 90 percent pure silver.