“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,” Dr. Drang blogs.
“How can this be? It’s because Apple’s gold is a metal matrix composite, not a standard alloy,” Dr. Drang explains. “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.”
“The karat measure of gold is based on the mass fraction. One hundred grams of 18k gold has 75 grams of gold and 25 grams of other material. If that ‘other material’ is a low-density ceramic, it takes up a bigger volume than if it’s a high-density metal. Because the casing of a watch is made to a particular size (i.e. volume), not to a particular weight, the Apple Watch will have less gold in it than an 18k case made of a conventional alloy,” Dr. Drang explains. “The patent makes it clear that saving gold is one of the goals of the process.”
More info and a bunch of math in the full article here.
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