they're using 18 karat gold which is 75% gold and 25% alloy which makes it more durable than pure gold (24k).
I found this article interesting, but I don't know enough about this stuff to fact check it.
http://www.slate.com/blogs/moneybox/2015/03/09/apple_watch_edition_the_company_found_a_trick_to_use_as_little_gold_as_possible.html