18 ct gold is 75% gold and the rest is silver and copper. 14 ct gold is 58.5% gold and the rest silver and copper. There is no nickel alloyed, which many people are allergic to, but of course you can be allergic to other metals like silver and copper.
http://www.sp.se/sv/units/weightsandmeasures/Assayoffice/Sidor/St%C3%A4mplar.aspx