Sky Gemstone

Sky Gemstone is a treasure.

Source
Sky Gemstones can be obtained as a sidequest reward for A Feline Feast and as collectibles throughout the world.
 * Tenebrae
 * Dungeon mazes in chapter 15

Quests
Sky Gemstones are needed for the quest A Better Circular Saw to upgrade from a Circular Saw to a Circular Saw Plus.