Marble Block

Marble only sits in the marble caves, so stocking up means finding one and stripping the cavern. Before the Wall of Flesh that is quiet work. After it, the same caves fill with Hoplites hurling javelins and Medusas that lock you in place from across the room the moment they have line of sight, so mine with blocks between you and the open air or clear the biome out first.

The raw block is rough and gray. Smoothing it is what unlocks the furniture set and the clean white look most builders are actually after, so haul back far more than you think you need. Going back for a second load is a worse trip than the first one.