The first discovery of diamonds by humans.

Diamonds formed billions of years ago in the Earth's crust; however the lust of humans for the beautiful gem began when they were discovered in India, in around the 4th century BC.

Since their discovery, diamonds were traded along the Silk Road to the rest of the developing world - where they were variously told to be the teardrops of the gods, lightning made solid, and supernatural talismans to ward off evil.

The Prince of Stone came into being when the veritable prince of stones, the diamond, fell into human hands, inspiring millennia of jealousy, violence and intrigue for its sake.

A bit of a spoiled brat, Cas likes luxury, and pines after a luxury greater than that which can be found in his rather utilitarian kingdom. Because of this, he's sort of an absentee ruler, neglecting his responsibilities out of selfishness and a childish dislike for a land he thinks is boring. 

He's very focused on things that he doesn't have, and probably wouldn't be happy even if he had them.

He's slightly vain, as well, and very prim and proper. He likes manners and dislikes "commoners" (although they kind of fascinate him at the same time).