If the metal is "white" that is, silver colored, then it could either be platinum, (although i usually see 980 in most platinum) or some type of close-to-fine silver.(the stamp for sterling is always 925) or maybe even some kind of palladium.
if the metal is "yellow" then i don`t know...
traditionally, jewelry grade karat gold translates like this;
750=18 karat
585=14 karat
417=10 karat
the three digit number is the percentage of actual pure gold present in the metal, eg.
750= 75% pure gold in 18k (the rest is alloy)
so.. if the metal is very yellow, it could be some type of highly refined 22 or 24k.