Prismatic Shard is one of the many minerals in the game. It is one of the rarest items in the game and it can be found in iridium nodes, omni geodes, gem nodes, mystic stones, and inside fishing treasure chests. It may be dropped by Mummies, Shadow Shamans, Shadow Brutes, Wilderness Golems, or Serpents.
You can take the Prismatic Shard to the Deserts and exchange it for the Galaxy Sword.
The selling price is 2,000g (2,600g with gemologist)