The Prismatic Shard is one of the rarest minerals in Stardew Valley, worth a decent chunk of change and useful in many ways. While you will need to find one to complete the Museum collection, it might be a while before you actually get one. There are a few different places you can find a Prismatic Shard, although many of those avenues have very low odds of it actually appearing.
How to get Prismatic Shard
The best place to find a Prismatic Shard is in the Skull Cavern. There is a small chance to find the Prismatic Shard in mystery boxes and fishing treasure chests, but mining in the Skull Cavern covers many of the other possible ways for it to drop.
It has a decent chance to appear in a Mystic Stone, which can appear in the mines post-floor 100, the Skull Cavern, and even the Quarry. It has a small chance to appear in Omni Geodes, as a drop from certain monsters, or in Iridium Nodes. Omni Geodes and Iridium Nodes can be found in large quantities in the Skull Caverns and the monsters that can drop it also appear more frequently there. Because the Skull Cavern contains multiple different ways to have a chance at dropping it, that makes it the best place to try and find them.
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Prismatic Shard uses
One Prismatic Shard is needed to complete the Museum collection. You can also trade a Prismatic Shard for a Galaxy Sword in the Calico Desert. In the northwestern part of the desert, there are three golden rods in a triangle formation. Stand in the middle with a Prismatic Shard in your inventory and it will become a Galaxy Sword, a powerful weapon. Getting the Galaxy Sword also adds a few other Galaxy Weapons to the Adventurer’s Guild shop.
Beyond that, the Prismatic Shard has two major endgame uses once you have completed either the community center bundles or the Joja Mart path. It is needed to complete the Movie Theater bundle and can be used to enchant tools with different boosts in the Volcano Dungeon on Ginger Island.
While the final two uses won’t open up until you get pretty far into your playthrough, it’s still a good idea to save at least a few Prismatic Shards. They do sell for 2,000G if you want the money, but you won’t have to get lucky and find more later on if you save a few.