Stamped, dimensional metal piece. Cold, rolled steel, non-magnetic. Colour them, stack them, mix and match them. Center hole. These are unsealed will rust if let them get wet - on the other hand - they are fun to deliberately patina. If you don't want them to change colour - a little coat of spray sealer does the trick.
Made in U.S.A.. 16mm x 16mm x 5mm By the each.