>>212487858 (OP)Because the jewel was from Caledon so I suppose it reminded her of him so she threw it to the sea as some symbolic way of saying goodbye to him for good (like "I'm not keeping this thing, it's not mine, it belongs to Caledon, and despite I'm aware of it's monetary value I don't need it and I don't want it").
And yes, Caledon went bankrupt a while after surviving the Titanic and committed suicide. Maybe if he had the jewel he would never have fallen into poverty. But in any case Caledon himself was the one that put the jewel into Jack's pocket to accuse him of stealing it (and Jack got beaten up by that asshole friend of Caledon).
And yes, the whole work team that rescued the paintings from the rests of the Titanic underwater and stuff could have gotten dirty rich with the jewel. But it technically was Rose's possession. She did what she considered best.
I see nothing good or wrong with it.