Brass. Black version is agate . Mustard version is bubble bee. Stone color can vary due to its natural origin . Size XS=T0= 51.5mm circumference= 16.4mm diameter . Size S=T1= 53mm circumference= 16.88mm diameter . Size M=T2= 55mm circumference= 17.52mm diameter . Size L=T3= 57mm circumference= 18.15mm diameter