Source: Fizban's Treasury of Dragons

Wondrous Item, Legendary (Requires Attunement by a Spellcaster)

While you are holding this gem, you can use it as a spellcasting focus for your spells.

The gem has 3 charges and regains all expended charges daily at dawn. When you cast a spell while holding this gem, you can expend up to 3 charges to ignore the spell’s material components with a gold piece cost, up to 500 gp per charge expended.

When you finish a long rest, choose a spell from any class list. The spell you choose must be of a level you can cast. You know the chosen spell and can cast it with your spell slots of the appropriate level until the end of your next long rest.