About the quality of gold:

Gold is a highly sought-after precious metal. It is dense, soft, shiny and the most malleable and ductile substance known. Pure gold is usually alloyed with other metals such as silver or copper to alter its hardness and ductility. Gold alloys are expressed in karats, such as 22k, 18k, 14k and even 10k. The higher the number of karats, the greater the value. These gold alloys obtain unique colour characteristics from the combination of metals in the alloy.

James Robson uses no less than 18k gold as well as other high quality precious metals in his creations. 18k gold is 18/24ths, or three-quarters pure gold.