Black anodised alloy.
Various sizes;
22.2 - 25.4mm - for BMX to MTB stems 
26.0 - 31.8mm - for Standard Road size to 31.8mm oversize 
25.4 - 31.8mm - for Standard MTB size to 31.8mm oversize 
31.8 - 35.0mm - for MTB 31.8mm to 35.0mm oversize +