18ga nickel silver and enameled copper wire, 3.5mm ID rings.
Each of these has apporximately 750 rings in it. These were a gift for my brother. He still needs two more, one of which I've finished, and the other I intend to do by Christmas.
These take forever. Ok, so I guess it's not too long, but each ball was taking me about 8 hours of solid work. I think the latest one I finished only took me about 6, but that's still quite a while for something so small. When finished, they're only about 5.5cm in diameter (~2 1/4" for if you prefer English measurements).