He knew all about this other group and the surgery Ben wanted. Yet he chose not to say anything to anyone.

WHY would he keep this info to himself?

To protect himself and Nikki? I guess in the end, even though he kept the diamonds a secret to help or save his relationship with Nikki, he was just a selfish guy. If he just spoke up, many things could have been avoided.

1. Because then he'd have to explain that he knew of The Pearl, but never told anyone. Jack and the losties wouldn't be too appreciative of that.

2. Because when news of The Pearl came out, Nikki would know he'd gone there in the dead of night. There would be only one reason he'd go down a dark tunnel to some strange place in the dead of night--he had the diamonds. He'd be found out.

3. Because HE still wanted the diamonds.