…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.
This patent, filed in December 2014, is an incredible find. It basically means that Apple can achieve 18K gold using much less pure gold (24K), than a typical 18K gold watch. If true, it could mean that the Apple Watch Edition could be much less than some have estimated.