The J block is an extremely rare block found only in survival worlds.
When mined, J drops itself.
The J block has no practical use in survival, though it serves as a trophy to show how long a player has spent mining in a world. When the player shift-clicks on the J block, the screen will turn red and this text will appear:
I'm quite useless.
|1.17||Oct 12, 2011||
|1.26.4||May 31, 2014||
|1.27||Nov 13, 2015||
- There is a J block in the background of the Mine Blocks website