Frozen Coin

Frozen Coins are objects appearing infrequently throughout the Super Mario series. In all games they appear in, they are coins encased in solid ice that can be melted by fire to be collected.

Super Mario Bros. 3 / Super Mario Advance 4: Super Mario Bros. 3
Frozen Coins first appear in Super Mario Bros. 3 at the end of World 6-7, and then appear throughout World 6-10. The player can melt the ice and collect the coins within by using their Fire form's fireballs.

Frozen Coins also appear in World-e of Super Mario Advance 4: Super Mario Bros. 3, first in a bonus room of Slip Slidin' Away with the level's first Advance Coin, then throughout Ice Cubed, where they play a more prominent role in the level's completion.

New Super Mario Bros. Wii
Frozen Coins return in New Super Mario Bros. Wii, where they appear in World 9-7. In this game, they can also be melted by fireballs from a Fire Piranha Plant or a Fire Bro.

New Super Mario Bros. U / New Super Luigi U / New Super Mario Bros. U Deluxe
Frozen Coins also appear in New Super Mario Bros. U in the level Piranha Plants on Ice, where they hold up Ice Chunks until melted by Fire Piranha Plants. They also appear in New Super Luigi U in the level Slippery Rope Ladders, where they function simply as obstructing platforms. Notably, both games have Frozen Coins appear slightly wider and with more ridges.

Frozen Coins reprise the aforementioned roles in New Super Mario Bros. U Deluxe.

Super Mario Maker 2
Frozen Coins were added in the version 2.0 update for Super Mario Maker 2, exhibiting the same behavior as in previous games. They can now be melted by any fire-based course element with the exception of lava, and the melting process is noticeably faster as well. Exclusively in the New Super Mario Bros. U theme, the spells used by the Koopalings can melt Frozen Coins.

When a P Switch is activated, the Frozen Coins turn into frozen versions of Brick Blocks (Rotating Blocks in the Super Mario World style). Frozen Brick Blocks, unlike their normal counterparts, cannot be broken by Super Mario.

Super Mario Maker 2

 * Play Nintendo: "These coins are surrounded by a block of ice and can only be released if melted by fireballs or other fire elements, including a certain angry sun."