Arcane Crystal

A pinkish-purple crystal that has a slight warmth to it.

A caster can use a spell slot to store it into this crystal. However, the limit of of the size of the spell slot that the crystal can hold is proportional to the crystal's mass. For example, a crystal that is 8 oz would be able to hold a 3rd Level spell slot. The user can then spend an bonus action to regain the stored spell slot.  If a user tries to store a higher spell slot than the crystal can hold, the crystal explodes into glittering dust.

The crystal can be put in an item for easier access, however it doesn't grant any benefit to  the item.