Cool Words: Dancing Diamonds

Wiki:

Shock diamonds (also known as Mach diamonds, Mach disks or dancing diamonds) are a formation of shock waves in the exhaust plume of an aerospace propulsion system, such as a supersonic jet engine, rocket, ramjet, or scramjet. It is formed when the supersonic exhaust from a nozzle is slightly over or under-expanded, meaning that the pressure of the gases exiting the nozzle is different from the ambient pressure. A complex flow field results as the shock wave is reflected at the free jet boundary and a visible diamond-shaped pattern is formed which gives the shock diamond its name.