These jump rings are great quality, sterling silver (not plated).
For your reference: .64mm is 22 gauge .76mm is 20 gauge, 1mm is 18 gauge, and 1.27mm is 16 gauge.
Listed is the diameter and then the guage size of the wire that the jump ring is made out of.
All of this easy reference information can be found in The Jeweler's Bench Reference by Harold O'Connor or in Tim McCreight's Complete Metalsmith. 2 great books to have on hand.
The newest jump rings are heavier gauge for strength and aesthetics. Make your own chain, extenders, and use to solder onto a surface for applique effects!