Large double (split) base metal jump rings, 12mm outer diameter (O.D.), 10.6mm inner diameter (I.D.) diameter, 0.7mm thick. Work like a keyring, you need to 'wind' onto them, very secure.
With regards to the split/double jump rings - they aren't as strong as some of the other jump rings available (eg the 'strong jump rings'), using tools will often bend them out of shape. Instead, add the component directly to the ring - eg twirl a headpin/eyepin directly on. If that isn't an option, use your nails to twist on the component, like you would a keyring.
Available in bright gold only. 4 gram pack contains approximately 16 rings.