A friend of mine game me some old jewlery to sell on eBay for her. She doesn’t know if they’re 10K, 14K etc. The only thing I can make out on most of them are the numbers 585, and one looks like it says .73. What is all that jazz?
It’s a European marking system. It’s basically describing the percentage of gold per thousand. So .73 is basically 73% gold (or 18K) and .585 is about 14K (14/24) by our standards here in the USA.
I’ve heard that taking a bath is a good way to solve this problem 
Eureka!!! 
Except… that tactic really only works for disproving that a piece of jewelry is solid gold… or if you know the specific density of the only other ingredient that the gold is alloyed with (which would have to be different than that of gold.)
Otherwise… well, given that a particular object has 85% the density of gold, where do you go from there??
I was thinking (to the extent I thought out a throw away joke post) that gold used in jewlery was usually alloyed with silver. A quick google doesn’t confirm that though, so it may be that greeks in bathtubs render us no help in this case.
Gold that is marked “.750” is 18K. Period. 18K is not marked in any other way.
.585 is 14K.
.417 is 10K(mostly US).
If you truly have something marked “73” then I don’t know what it is.
And the alloy used with gold is copper. Sometimes a very small trace element to make the color different. But the main alloy is copper.