This is a great post from Dr. Drang in which he details how Apple may be formulating the 18-karat gold in the Apple Watch:
Apple’s gold is a metal matrix composite, not a standard alloy. Instead of mixing the gold with silver, copper, or other metals to make it harder, Apple is mixing it with low-density ceramic particles. The ceramic makes Apple’s gold harder and more scratch-resistant—which Tim Cook touted during the September announcement—and it also makes it less dense overall.
Because the karat level of gold is measured by the mass fraction, Apple’s 18k gold watches could contain much less gold than you might expect if it’s mixed with a low-density material rather than other high-density metals.
—Linked by Jason Snell