Glowing Mushroom

Breaking the plants growing on mushroom grass is the obvious source, but the giant mushrooms drop a pile when chopped down, and the vines and cattails hanging off mushroom grass each count as a separate plant, so a long curtain of vine is worth much more than it looks. Harvesting also yields Mushroom Grass Seeds, which is how you start a patch anywhere else in the world.

Only living plants add to whether an area reads as a mushroom biome. Mushrooms you place as blocks contribute nothing, and neither do mushroom walls or any of the furniture. This trips up people building a surface biome to house the Truffle: the check wants mushroom grass with plants standing on it, not a room built out of mushroom material.

Mushrooms grow from the underside of a grass block as readily as the top, so a farm built as a stack of thin ceilings produces far more per tile of grass than a flat field does. Flooding it helps as well, since submerged growth runs taller before you harvest it. Between the two, a small footprint keeps up with any crafting habit you have.

Raw mushrooms sell for close to nothing, so convert before you sell. Mushroom Candelabras are the simple early sink, and if you are paying for the torches that go into them anyway, routing the mushrooms into upgraded healing potions instead is slightly better money. In Hardmode, Shroomite Bars are worth far more than what goes into them and you want the bars regardless. Restoration Potions, Mushroom Torches, and the Mushroom Staff are the other places they go.

It places straight down as a solid block without being crafted first, and there is a full furniture line behind it. Painting the plants changes the color of the light they throw, which is the cheapest colored lighting in the game and works on the giants and the vines too. Set three blocks in an upside down T, hammer it, and paint it, and you have a passable rock; the middle block has to connect to what is below it for the shape to hold.