Hey Andrea, new here as well....
If your wanting to do a wire thru the nugget, looped on either side, I'd use the 18, then forge the loop flat, it will work harden it, and prevent it from popping open.....I do this with sections on some of my necklaces....IE: 6mm copper bead, 12 mm round turquoise or nugget ,and do these in sections of 3 ......so the 18 g with forged loops are holding 5 copper beads, and 3 hefty chunks of stone....none have been sent back yet due to failure, and Ive been at this a long time
Good luck
Steve