The Ring Tower in black onyx

The same puzzle as our wooden piece of the same name, with every part cut from black onyx instead. Each part is shaped from one piece of stone and polished, and every edge and corner is rounded so the stone does not chip where the cord pulls across it.

Black onyx is a fine-grained quartz polished to a deep, even black. At 6.5 to 7 on the hardness scale, everyday use does not scratch it.

It is a fine-grained or fibrous stone. Its tiny grains lock together and stop a crack from running, so a diamond tool follows our CAD file cleanly and the finished piece resists chipping.

The cord is the same as on the wooden piece, so the solution is the same. The piece comes in nine stones and their natural colours: pick one above and the price follows it.