What is density?
Short answer: Density is mass per unit volume, and it is one of the most useful checks on whether a piece is the metal it claims to be.
Pure gold is about 19.32 g/cm³, silver 10.49, platinum 21.45 and palladium 12.02. A piece with the right weight but the wrong size is a warning sign. Density is a screening check, not a certificate of purity.