The registered trade mark shows the differences in quality. For example: "14k" means 58.3% is pure gold, "18k" is 75% pure gold. When you see the stamp "14k" this means that 14 of 24 parts are pure gold. The remaining 10 are alloy. Alloy is a mixture of several different metals added to strengthen, or in some cases to change the color to a whiter tone or a rose tone. Platinum is the hardest and most rare of all metals used in jewelry making. It can be marked with "PLAT," or "950 PLAT." The United States requires the manufacturer's trademark to be stamped on the piece to provide authenticity.