You know everything there is to know about Diamonds, right? Well let’s see if I can stump you. What is a shock diamond? No idea, huh?

Well it’s a trick question. Because it isn’t really about gemstones. Shock diamonds, otherwise known as mach diamonds, per Wikipedia, 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.

These shock waves are 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. The visible diamond-shaped pattern, that gives the shock diamond its name, is formed.

For more information, read this article at Aerospace Web.