Blackstone chain is an ultra-strong used independently approved 10mm metal chain with hardened steel construction. This Heavy Duty 10mm six-sided square-linked chain links made of 2t HARDENED MANGANESE STEEL for ultimate strength making it ideal for outdoor use; it also features a nylon sleeve against scratches to bicycle motorcycle surfaces. Blackstone Security Chain Features Six-sided shaped links much more effective against attacks from bolt cutters saws hammers and chisels. It passed the Golden Bicycle Test approval which stands for the best quality.