Bent Stamp 18K - SFC Tools - 55-272 This "18K" stamp can be used to mark 18 Karat gold. The stamp is cut into 0.25 (7mm) hardened steel square stock. The size of the "18K" is 1.0x3.0 mm. The stamp is 3.38 (85 mm) long and has a beveled top for striking with a heavy mallet.