From the Super Mario Wiki
Meta Crystal is a stage in the game Super Smash Bros. It consists of one small platform hovering above a larger one. It is not unlockable, and it is only used for one battle against Metal Mario in 1-Player mode. Even though the stage is small, it's hard to knock Metal Mario off of the stage because of his weight. The background is made up of metallic crystals and a red wire fence. These objects can be found in the Hazy Maze Cave and the Cavern of the Metal Cap in the game Super Mario 64. This stage is the only one in the entire series not to be remade in another form or replaced in the sequels. All others are either updated (i.e. Battlefield and Final Destination) or replaced with similar counterparts (series-based stages), as the purpose of fighting metal characters is taken by Battlefield in Melee and then by any random stage in Brawl.