Stone Block

Stone fills most of the Underground and Cavern, so it is the obvious material to grab when you need thousands of blocks. Skip it for anything you want to keep. Once a world turns Hardmode, the evil biomes and the Hallow both convert plain stone on contact, and a base made of it will rot inward from the edges. Anything you craft out of stone is immune to that. Stone Slabs keep the same flat gray and cost you nothing extra per block, Gray Bricks cost a little more, and neither will carry an infection, so a wall of either doubles as a barrier.

If you specifically want the untouched stone texture, Active Stone Blocks look identical and cannot be converted. Wire is the catch there, though gutting a natural trap corridor supplies plenty of it for free.

Mining is the slow way. Rock Golems and gem trees both drop stone, so no world ever truly runs out. The fastest early stockpile is the Merchant: house him near the Aether, buy Furnaces by the stack, and drop them in Shimmer to break them back down. Keep some on hand if you plan to grow gem trees, since stone is the only soil a gemcorn will accept.