Standard US wire thickness system. Smaller number = thicker wire. 10 AWG is thick structural; 30 AWG is fine beading wire.
AWG (American Wire Gauge) is the standard US system for specifying round wire thickness. The system runs backwards: LARGER numbers are THINNER wire. 4 AWG is ~1/4 inch thick structural cable; 30 AWG is hair-thin beading wire.
The formula is d = 0.005 × 92^((36 − AWG) ÷ 39) inches, where d is diameter. Each 6-step decrease in AWG roughly doubles the diameter; each 3-step roughly doubles the cross-sectional area.
10 AWG
0.1019 in (2.59 mm) — cuff bracelets, heavy structural
18 AWG
0.0403 in (1.02 mm) — standard jump rings, ear wires
22 AWG
0.0253 in (0.64 mm) — wire wrap loops, fine chain