Shock diamond

A shock diamond, also known as a shock collar or Mach diamond, is a phenomenon that occurs in supersonic flows when a jet, such as that from a rocket engine, exhausts gases at a speed greater than the speed of sound into the surrounding air. This results in a series of shock waves that form a diamond-shaped pattern when viewed from specific angles. In essence, the shock diamond is a visual representation of the pressure changes in the flow surrounding the jet.