Very Rare Material

While holding the gem, you can use your action to crush it in your hand, creating a psychic wave. All creatures within 60 feet of you must make a DC 17 Wisdom saving throw. On a failure they take 6d6+4 psychic damage and become unable to use either their bonus action or reaction until the start of your next turn, depending on what they choose.

Each time you use this feature, the gem becomes more damaged and cracked, making it lose value and increasing the DC for crafting the Overmind Crown by 2.

The Gem is permanently destroyed after you use this feature for the fourth time. 
Can be used to craft the Overmind Crown with a successful DC 25 Jeweler’s Tools check, also requiring 7 500 gp in resources and 7 days of crafting time. If you fail the check by 10 or less, you lose only the resources, if you fail by 11 or more, the Mind Gem is destroyed as well.