Black zirconium is a metal with very interesting properties, made by oxidising a zirconium wedding ring to blacken the exterior. Zirconium is metal with similarities to titanium, and is used in many industries including nuclear reactors and space vehicles due to its resistance to heat.
Black zirconium has a tough surface of a ceramic-like layer that is highly resistance to wear.
The heat treatment of zirconium rings does mean that black zirconium rings are about 50% more expensive than plain titanium or plain zirconium. However the treatment does means the ring remain in its original condition for a long time.