Earlier quoted context omitted.
"18K Gold developed to be twice as hard as standard gold." Did they really say that? Ha! Who knew my 9K gold stuff was vastly superior to higher grades.
18K just means "75% pure gold", it doesn't say what the other 25% is and there are a lot of different alloys. Most commonly it's mostly copper, which ironically is what gives it the standard "gold" tint (white gold uses metals like silver instead). I assume they mean "we picked an alloy that's harder than most".
"crafted from 18-karat gold that our metallurgists have developed to be up to twice as hard as standard gold."