Rainbow Brick

The Rainbow Brick is a drop from the Rainbow Slime. These bricks cycle through various colors. The cycling colors are synchronized with the disco ball's lights. When the source of a waterfall is made of Rainbow Brick, the waterfall will also cycle through colors. The rainbow brick is the hardest obtainable block in the game, being the source from Rainbow Slimes which only spawn during Solar Eclipses.

Trivia

 * When picked up in the inventory, and as well as the item ID picture, the brick appears Gray, as animations of items probably cannot happen while being held with the cursor.