The "diamonds" are actually a complex flow field made visible by abrupt changes in local density and pressure as the exhaust passes through a series of standing shock waves and expansion fans.
The physicist Ernst Mach was the first to describe a strong shock normal to the direction of fluid flow, the presence of which causes the diamond pattern.
[2] The external compression is caused by oblique shock waves inclined at an angle to the flow.
This locates the first shock diamond, and the space between it and the nozzle is called the "zone of silence".
When artillery pieces are fired, gas exits the cannon muzzle at supersonic speeds and produces a series of shock diamonds.
The diamonds cause a bright muzzle flash which can expose the location of gun emplacements to the enemy.
[1]: 68 The jets travel at supersonic speed through a thin "atmosphere" of gas in space,[1]: 51 so it is hypothesized that these knots are shock diamonds.