Properties of 9k Gold

Hi All, I have a collection of vintage Opal jewelry - all handmade
pieces. One of the later pieces this gentleman made is an 18x14
treated matrix opal set in what is supposed to be 9k dental gold. It
was made in 1945, while there was a moratorium on owning gold. I
have included a link to a photo. As you can see it has acquired a
black patina over the years, due no doubt to it’s higher alloy
content. It is currently a size 4.5 and I would like to resize it
up quite a bit.

Does anyone know the properties of this metal? I am assuming I
could use 10k for the stock, but don’t want to ruin it by assuming
anything. Any and all would be greatly appreciated!

Jim Marotti
Lancaster, TN