Gold Shell

A Gold Shell is a very rare type of Koopa shell, that seems to be so rare it only appears in two galaxies of Super Mario Galaxy. It only appears in the levels "Passing the Swim Test" and "Bigmouth's Gold Bait".

Super Mario Galaxy
When the player arrives at the Beach Bowl Galaxy, he learns that the penguins' Coach gave his students a "Swimming Test" where the students have to find a Gold Shell. He states that the penguin that finds a Gold Shell instantly gets an "A" score. When the player dives underwater, he sees that a penguin has already found a Golden Koopa Shell and it is also the last and only one there is. The player later swims up to the penguin who says that he won't give him the shell. However, the player can take the golden Koopa Shell from him by spinning and can take it back to Coach, who is so impressed that he gives Mario a "gold medal" (which is actually a Power Star). Coach later realizes that Mario is not a penguin and starts wondering if Mario really is one of his students. After Mario gets the Power Star, he will have to leave the Gold Shell behind.

In the Bigmouth Galaxy, the player finds a golden Treasure Chest that will only open if he brings back a Gold Shell. After swimming through a dangerous cavern in the darkness, the player must find the Gold Shell to bring back up, where it opens the chest, freeing the Captain of the Toad Brigade, who was trapped in the chest. He gives Mario a Power Star as a thanks for freeing him.

New Super Mario Bros. 2
Gold Shells return in the Nintendo 3DS game New Super Mario Bros. 2, where they are now the golden versions of the average Koopa shells. They are much more common here, as Gold Shells appear when Mario passes a Gold Ring. When a Gold Koopa Troopa is stomped, they retreat into their golden shell, and if kicked it produces a trail of coins behind it.

Super Mario 3D World
Gold Shells also appear in Super Mario 3D World, although they only appear in The Bowser Express. There is also no color code for the shells.