Crystal Ball

Crystal Balls are items featured in many games throughout the Mario franchise.

Super Mario Bros. 2
Crystal Balls are first seen in Yume Kōjō: Doki Doki Panic and its update, Super Mario Bros. 2, where it gains an animated red line. They are found at the end of most levels. Picking one up will open the Mask Gate and allow the player to proceed to the next level. They are usually guarded by Birdo. In the NES version of Super Mario Bros. 2 as well as Yume Kojo: Doki Doki Panic, Birdo can be seen holding the Crystal Ball in front of her during battle. In later versions, it seems to come out of her snout upon defeat.

Paper Mario series
They are seen in Paper Mario as the main magic tool for Merlon's family. There is an Ultra Stone variant. It is also seen with Merlon in Paper Mario: The Thousand-Year Door.

In Super Paper Mario, when Chapter 5 is complete, the player can go see Merlee at Flopside. She claims that she wants a Crystal Ball from Merluvlee, and wishes the player to go get it for her. After doing the task, Merlee rewards the player with a free charm and a key that opened a storehouse containing Piccolo the Pixl.

Other appearances
Madame Clairvoya uses one to see the future and other things in Luigi's Mansion. Mini-Marios are also encased in one as packaging of sorts in Mario vs. Donkey Kong. Another is used in Mario Party 6, in the Seer Terror minigame. Bowser uses it to see a player's "future," which usually is something bad. Occasionally, however, the player receives Coins or a Mushroom. Kamek used one in Yoshi's Island DS.