Gold Shell

A Gold Shell is a rare type of Koopa shell. It first appears in Super Mario Galaxy, where it is the objective of some missions. In later games, it can give Coins.

Super Mario Galaxy
In Super Mario Galaxy, the Gold Shell only appears in two missions: "Passing the Swim Test" and "Bigmouth's Gold Bait."

When the player arrives at the Beach Bowl Galaxy, they learn that the penguins' Coach gave his students a "swimming test" where the students have to find shells. He states that the penguin that finds a Gold Shell instantly gets an A. When the player dives underwater after speaking with Coach, they will see that a penguin has already found the sole Gold Shell. The player must swim up to the penguin, who, refuses to give up the shell. However, the player can take the Gold Shell from him by spinning, allowing them to take it back to Coach, who is so impressed that he gives Mario a "gold medal" (which is actually a Power Star), only to realize Mario is not 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 they bring 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 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 is stomped, they retreat into their golden shell, and if kicked it produces a trail of coins behind it.

Super Mario 3D World / Super Mario 3D World + Bowser's Fury
Gold Shells also appear in Super Mario 3D World and Super Mario 3D World + Bowser's Fury; once again, they are rare. They only appear in Koopa Troopa Cave, Sunshine Seaside, and Coin Express. They function similar to the ? Box and Gold Shells in New Super Mario Bros 2: the player gains coins for shell-dashing with these shells or throwing them; they give 30 coins before breaking. There is also no character color code for the shells.

Mario Kart Arcade GP DX
In Mario Kart Arcade GP DX, Gold Shells act just like Black Shells, but instead of being thrown normally like a Green Shell, they home in on the driver in front of the player, like a Red Shell, exploding upon impact. Triple Gold Shells can be unlocked and used as an item as well.

Nintendo Kōshiki Guidebook – Yoshi's Egg
金 上の4種類のこうらの成分すべてを含んだこうらだ. 味はすさまじくまずいが、体力を極限まで高めてくれる栄養がある. 良薬口に苦し、というアレである.

Gold ''A shell that includes all ingredients of the above 4 shell types. It tastes dreadfully unpleasant, but its nutrition will maximize physical strength. Good medicine tastes bitter, as they say.''

Trivia

 * According to the Japanese Shogakukan guide for the game Yoshi, the flashing shells are considered "gold" shells that combine all ingredients of the Green, Red, Yellow, and Blue Shells from Super Mario World, resulting in awful taste but incredible benefits.