Dr. Drang on his blog, Lean Crew:
Apple’s patent application is for a method that allows them to make 18k gold that has, on a volume basis, less gold than regular 18k gold.
How can this be? It’s because 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 is a huge deal. As I've mentioned before, the amount of gold in Apple Watch makes a huge difference in the overall cost of the device.