The Dirtiest Block

It hides in plain sight. Somewhere in your world a small handful of tiles are secretly this pet item, and they render exactly like the ordinary dirt around them, so mining at random almost never turns one up. The reliable method is chemical: spray a wide field of dirt with White or Yellow Solution and watch for what refuses to convert. Anything left unchanged inside a converted patch is what you are after.

World generation keeps testing random spots until it runs out of attempts, so a world can easily end up with fewer than its size theoretically allows. Grass also refuses to take root on the tile, which gives you a slower second tell on the surface: a bare gap in an otherwise green hillside.

Once summoned it just bobs along behind you as a cube of soil. Dye it and it takes the color, which is the only customization on offer and the only reason anyone bothers with it twice.