Clay Block

None of the three spreading biomes can cross clay. That makes it a containment material you can dig up by the thousand on the surface rather than hauling from a specific depth or crafting in batches. If you are walling off a section of world before Hardmode, this is the cheapest thing to line the trench with.

Brown paint turns it close enough to dirt that a barrier disappears into the terrain instead of reading as an orange stripe across your world. The same trick gives you patches of ground that look like dirt but will never grow grass, which is handy for paths and for keeping a build from going green over time.

It sits in the surface layer and thins out as you go down, so tunneling for it is the wrong approach. Walk the surface and look for the off-color patches in the dirt: the large deposits are visible from the ground without any digging at all. A Portable Kiln will turn what you gather into pots on the spot.