Are they usefull, or only money makers?

I'm not returning the diamond only for 2500 XP so I think I'll sell it.

Also, how long does it take for Antone to make an item... I've given him the pieces and now I gotta wait for him to finish.

Diamond you sell for cash. Silver Nugget and black obsidian you give to Ferro to make his custom items, and it's a 24 hour waiting period for your stuff to come in. Just go into his room and camp 3 times in a row. He won't mind if you crash on his bed.