Feed hungry critters in the Geode caves. Hungry critters show a treat icon above their heads.
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— Item Description
Critter Treats are a resource that can be crafted at the Crystallogy Workbench. There are two recipes for it in that workbench, one to make 5 treats and the other to make 100 treats. Throwing these at a critter with a icon above its head will give you Lesser Crystal Caches, with a rare chance at obtaining a critter-specific Companion Egg.