For piggie 
http://leancrew.com/all-this/2015/03/apple-gold/
Apple's patent application: http://pdfaiw.uspto.gov/.aiw?PageNu...f=G%26l=50%26s1=20140361670.PGNR.%26OS=%26RS=
http://leancrew.com/all-this/2015/03/apple-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.
Apple's patent application: http://pdfaiw.uspto.gov/.aiw?PageNu...f=G%26l=50%26s1=20140361670.PGNR.%26OS=%26RS=