Crystal Shard

Shards sprout on the sides and undersides of the hallowed block family, which in practice means the underground hallow is where you go for them. They keep coming back after you harvest, which puts them in the small group of hardmode materials you can never actually run out of. If you would rather fish than travel, Divine Crates carry them often enough that a session on the lake doubles as a shard run.

Growth checks two things. The block feeding the shard has to sit at cavern depth or lower, though it does not have to be inside a genuine hallow biome, so a hauled-in stack of pearlstone works fine in a room you dug yourself. The spot itself has to be uncrowded: if two other shards already sit anywhere in the wide square around it, nothing new appears there. Water does not block growth and neither does a background wall.

That crowding check is the whole design problem for a farm. A solid slab of pearlstone chokes itself, because every tile on its face competes with its neighbors. Thin rows with gaps between them keep almost every tile eligible. Long single-thickness lines with clear space above and below will out-produce a dense block built from the same number of tiles.

They give off a faint light of their own, the same trick demonite and crimtane use, so the easiest way to find a wild patch is to stop lighting the cave. Kill your torches, let the dark settle, and the walls do the work for you. Now and then a growth comes up as a Gelatin Crystal instead. That one is worth stopping for rather than swinging past.

Most recipes are one and done. The phasesabers are where the total balloons if you want every color rather than a single blade, and the Rainbow Phasesaber is a project on its own. The only recipe that keeps drawing on the pile forever is Super Mana Potions, which is the reason a magic player keeps a farm running long after the weapons are finished.

Once harvested they can be stuck on a floor, a wall or a ceiling anywhere in the world, hallow or not. The restriction on where they grow does not apply to where they go. Treat it as decoration, since they are too dim to light a real room, and paint recolors the crystal itself while the glow keeps the color it started with.