Source: Tasha's Cauldron of Everything

Wondrous item, rare (requires attunement by a sorcerer)

This crackling crystal contains the essence of an elemental plane. As an action, you can attach the shard to a Tiny object (such as a weapon or a piece of jewelry) or detach it. It falls off if your attunement to it ends. You can use the shard as a spellcasting focus while you hold or wear it.

Roll a d4 and consult the Elemental Essence Shards table to determine the shard's essence and property. When you use a Metamagic option on a spell while you are holding or wearing the shard, you can use that property.

Elemental Essence Shards
d4	Property
1	Air. You can immediately fly up to 60 feet without provoking opportunity attacks.
2	Earth. You gain resistance to a damage type of your choice until the start of your next turn.
3	Fire. One target of the spell that you can see catches fire. The burning target takes 2d10 fire damage at the start of its next turn, and then the flames go out.
4	Water. You create a wave of water that bursts out from you in a 10-foot radius. Each creature of your choice that you can see in that area takes 2d6 cold damage and must succeed on a Strength saving throw against your spell save DC or be pushed 10 feet away from you and fall prone.