Pink Prickly Pear

A sword swing leaves it sitting there. You need a pickaxe, which catches people out, because nearly every other surface plant pops off any hit at all. Pink Dye is the only thing it turns into, so strip a desert in one pass rather than walking past them and coming back.

It only appears on the top segment of a cactus out in the open desert, and the game checks the ground around that cactus before it will place one. The check covers a tall column centered on the plant, reaching a short way out to each side and much further down than up. One block of ebonsand or crimsand anywhere inside that column cancels growth outright. The two corners diagonally beneath the cactus also have to be open air, so a cactus wedged into a slope or buried against terrain never produces anything.

You will sometimes find a pear in evil colors, and it did not grow that way. Those are ordinary pears that got converted after the fact, either by biome spread creeping across the sand or by someone firing a solution over the area. Hallowed cacti are the exception and do grow them from scratch.

A Spelunker Potion tags them exactly the way it tags ore, which is far quicker than scanning cactus tops by eye. Do not plan on decorating with them either. Of every dye material you can find growing in the world, this is the only one you can never put back down once it is in your inventory.