Amethyst Stone Wall

Gem caves are lined with these, and breaking one hands you nothing. The generated panel is scenery with no item behind it, so your only supply is the crafting bench. Amber is the odd one out in the other direction: it never forms as an ore vein, which means no cave anywhere in any world has amber walls in it. Every Amber Stone Wall in existence was placed by a player.

A panel you place yourself counts as safe and stops enemies appearing in front of it, which the world's own copies do not. Hollow a base out of a gem cave, leave the original walls up, and the room keeps spawning monsters at you no matter how much you light it. Swapping them out for the crafted version, panel for panel, keeps the exact look and shuts the spawns off.