18k, 21k or 925 silver: what the numbers mean
The numbers used for jewellery describe how much precious metal is in the alloy. Knowing what they mean makes it easier to compare designs.
Karats describe gold
Gold purity is counted in parts of 24. 18k gold is 18 parts of 24, or 75%. 21k gold is 21 parts of 24, or 87.5%.
925 describes silver
Silver is counted in parts of 1000. 925 silver is 92.5% silver, with other metals added to the alloy.
Why purity alone is not the whole picture
Purity tells you how much precious metal is in each gram. Weight tells you how many grams there are. Compare the two together, and ask how the finished alloy looks and wears for the piece you have in mind.