What was the diamond in the Queen's crown stolen?

Excuse me, but I couldn't help but notice the mention of a diamond being stolen from the Queen's crown. Can you please elaborate on this? Was it a particularly valuable or significant diamond? Was there any security measures in place that were breached during the theft? How did the authorities respond to this incident? And most importantly, has the diamond been recovered or is it still missing? I'm quite intrigued by this development and would love to hear more about it.