Wondrous Item, rare (requires attunement by a Long rest )
It is raw natural magic in crystalline form. Mined from the inside of the Thayvian Mountains. Thought to be made of prehistoric coral reefs deposits transformed to a metamorphic rock from the heat and pressure in the mountains. Roll a d6 to see if you get 1 natural resistance or immunity: fire, cold, lightning, thunder, acid, poison. Wild magic becomes a 50% chance.
Notes: Immunity: Cold, Bonus: Spell Attacks, Bonus: Spell Save DC, Bonus: Spell Attacks, Long rest