In gold I know that if it is a certain karat then it is that many parts out of 64. (Like 10k is 10 parts gold to every 64 parts.) But what I don’t understand is what comprises the rest of the “gold.”
IIRC, 24k is “pure” (not completely, but close enough). The rest is usually copper, nickel, or silver, depending on the circumstances.
Actually, it’s parts per 24. So 24 carat gold is pure gold. That’s rarely used in jewelry because gold is so soft.
IIRC, silver is a common “filler” metal for the missing carats.
I think that gold karats are measured out of 24 parts, not 64…
Anyway, the metals gold is most often alloyed with are silver and copper, because they have similar properties.
Copper, silver, nickel, zinc, platinum, chromium, cobalt, tin, zinc, indium and palladium are commonly used in the most popular alloys of gold. White gold may be alloyed with zinc, nickel, palladium, copper and/or silver in varying proportions, and is rhodium-plated for extra shiny whiteness. Increase the proportions of silver in the mix and you get green gold. Increase the proportions of copper and you get red/rose gold. Yellow gold is indeed primarily alloyed with silver and copper. You can alloy 23k gold with a very small amount of titanium for a fairly hard alloy while retaining the buttery color of pure gold, and there’s some other funky combos besides. Adding aluminum gets a purply-plum colored gold, but I hear it’s a bear to work with. Gold and steel can make blue gold, which is hardly ever seen. “Hamilton” is a rose-yellow alloy of gold. Probably TMI, but I like gold. 
Gaudere, is there a trade or common name for this alloy of gold you mention? It sounds really cool!
Una
Not that I know of. I’ve only seen it used in a very few modern one-of-a-kind pieces in fine jewelery design shows. I thought it was pretty cool, too. Some places make titainum and gold two-tone pieces, and plain titanium wedding bands appear to be becoming popular. I don’t know that I’d get a pure titanium ring, though; I’ve been too well-trained that light metal==cheap and it just wouldn’t feel right.
No-one ever said you were cheap that’s for sure
Titanium freaks can visit Titanium Wedding Rings to see some gold inlay rings. If you want that ring that Ed Harris wore in The Abyss, the one that looks like a short length of pipe, it’s here.
I’m a weird and exotic metal fan. If I could find an iridium, osmium, tungsten, or some sort of odd metal ring that was not overly outrageous I would buy it. Just because it would be cool.