Yellow gold ring of 18 carats set with an oval amethyst

Yellow gold : Yellow 18K gold is an alloy contianing 75% of pure gold, whereas 14K gold contains only 58,5% of pure gold.
Amethyst : Amethyst is a popular form of quartz that ranges from a bright to dark or dull purple color. Heated the color of the amethyst changes to yellow and is then called citrine.
Gold : Because of the softness of pure gold, it is alloyed with silver, copper or palladium. The proportion of these other metals give yellow, white gold. Without precision, the gold used in jewelry is 18 K, corresponding to 75% pure gold.