12mm average size. Beautiful herkimer diamond prong set pendants. No two alike, check out the enlargement to see the open back side.The bail is sterling silver with a touch of rhodium which helps prevent tarnish. This pendant is made with an AA grade Herkimer Diamond. Herkimer Diamonds are quartz formations which because of their brilliance and diamond shape have been given the name. Mined only in Herkimer, NY. Herkimer diamonds help release energy blockages and also help enhance dream recall. Really nice!