Flesh Block

It looks like the biome and it does not act like it. Place it next to clean jungle or a hallowed patch and nothing spreads, so you can build a full themed room in the middle of an otherwise tidy world without digging quarantine trenches. That is the main reason to convert the stone rather than build with the stone directly.

Hammering cycles through several tile textures rather than shaping slopes, and the textures are the point. Line the right ones up and you get a small version of a certain boss face, which is one of the more common things people build out of it. It also squelches when struck and puffs red particles when you land on it, so a floor made of it does more work than a wall.

Some of the alternate seeds place it naturally, including up on floating islands, and in those worlds it counts toward making a crimson biome. That is the one case where the no spread rule stops being harmless: enough of it in one place will register as the biome even though it never crept there on its own.