Crystal Ball

Super Mario Bros. 2
Crystal Balls are items that have been featured in many games throughout the Mario series. They are first seen in Yume Kōjō: Doki Doki Panic and its update, Super Mario Bros. 2. 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.

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.