A 750 mark on jewelry means that the metal alloy used to make the jewellery contains 75 percent gold or another precious metal. Most precious metals used in jewelry are alloys because the addition of other metals improves hardness and durability. It also changes the color of the metal.
Gold jewelry with the 750 hallmark consists of 18-karat or 18K gold and bears a European hallmark. In the karat system used in America, pure gold contains 24 karats of gold, so the whole comprises 24 parts.
Replacing six parts with alloy metals yields a purity level of 75 percent. Adding palladium or nickel to yellow gold creates white gold. Adding copper produces rose gold.