þungsilfri (-s) (platinum)

 Apart from "gylfri" and "hvíyagull", there's another possibility for platinum: þungsilfri. It would be absurd to call the heavier chemical brother of silver, gold, "heavy silver", because in this case it would be inappropriate to call a yellow metal "a kind of silver". Add to this the fact that the Spanish called the element after "silver" (little silver, plata + -ina) and constructions like "þungsilfri" might be a good alternative for "hvítagull" (white gold).