Crystalline Fragments
Crystalline Fragments spawn on Crystal Lattice Seekers and on the ground in the Prism of Light.
They are part of a quest that can be obtained from Jeremiah the Iron Worker in the secure area of the Prism of Light. The quest is called "Crystalline Fragments". You will need to collect 10 Crystalline Fragments, and in return you will receive a Craftsman's Satchel.
They are also one of the ressources an alchemist may use to craft Crystal Dust which is used for Gorgon Lenses at Medusa.
Last but not least, they are a nice light source when locked down in a house.
...So I tried this and locked some down, it got dark, they did nothing... ???