Cheap Plastic. Some of my lug nut caps broke immediately when I put them on the lug nuts. Some of the Lug Nut caps seemed less noticeable to them than others. I'm not sure if this is a casting problem in their factories, but they shouldn't break that easily. I was looking forward to using them as they look good but the quality leaves a lot to be desired. I've tried contacting the manufacturers and I'm hoping they will respond with a replacement part or a refund.