Complex sentence: It is now known that this rare blue diamond was once part of the stolen Royal and Imperial crown jewels of France, known as the French Blue, which was stolen along with most of the French Crown jewels in September of 1792.
Split into simpler sentences: It is now known that this rare blue diamond was once part of the missing Royal and Imperial crown jewels of France, known as the French Blue. It had been stolen along with most of the French Crown jewels in September of 1792.