From the Super Mario Wiki
“Huh?!?! You want this, amigo? This little rock over here? Are you serious? Well, that's no big deal... Here, take it. What do I need one or two extra gems for? I did not really like that one, anyway.”
The Sapphire Star is the fifth Crystal Star in Paper Mario: The Thousand-Year Door. It is found in the Pirate's Grotto, as part of Cortez's treasure hoard. Cortez gives this Crystal Star to Mario when Mario tells the undead pirate he is not interested in his treasure, only the Star. Cortez hands it over willingly, stating that he did not even particularly like the Star. Acquiring the Sapphire Star teaches Mario the move Sweet Feast.