From the Super Mario Wiki
Crystal Blocks are a type of block debuting in Super Mario 3D World and appearing again in Captain Toad: Treasure Tracker. They are blocks made out of a purple crystal similar in appearance and function to normal Brick Blocks, in that they shatter when hit, unless they are hit by Small Mario. Unlike Brick Blocks, however, Crystal Blocks never contain items.
Crystal Blocks appear as course elements in Super Mario Maker 2, as well as in the background of the underground level theme in the Super Mario 3D World style, which itself is based on Koopa Troopa Cave.
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