Tungsten Carbide 8 mm (5/16 in) Comfort Fit Flat Wedding Band Ring w/ Black Brushed Center & Silver Beveled Edge (Available in Sizes 8 to 11) 8 mm (5/16 in) Comfort Fit Flat Wedding Band Ring with Black Brushed Center & Silver Polished Beveled Edge. Black Tungsten Carbide is completely cobalt-free and ranks next to Diamonds as the 2nd hardest material in the world. Elegantly packaged in a black velvet ring for gift giving. 100% Satisfaction Guaranteed.