These gemstones contain a crystallized bit of spellcasting magic. Different types of mind crystals exist, each with a different single-use effect.
When you cast a spell that has a casting time of 1 action while holding a mind crystal, you can modify the spell in a specific way. You can use only a single mind crystal to modify the spell, and you can't use a mind crystal and a Metamagic option on the same spell. Once you use a mind crystal, it becomes a nonmagical gem worth 50 gp.
Multiple variations of this item exist, as listed below: