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 found in the Pirate's Grotto and is one of Cortez's treasures in Paper Mario: The Thousand-Year Door. 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 didn't even particularly like the Star.
The Sapphire Star teaches Mario the move Sweet Feast, which costs five SP. As opposed to Sweet Treat, the player will simply have to hold left instead of repeatedly tapping it. Mario can gain much more HP and FP using this, but Poison Mushrooms are still a threat and appear more frequently.