Source: Phandelver and Below - The Shattered Obelisk

Wondrous Item, Rarity Varies

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 that has an effect described below. 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.

Careful (Uncommon). Choose up to three creatures affected by the spell. The chosen creatures automatically succeed on their saving throws against the spell.

Distant (Uncommon). If the spell has a range of 5 feet or more and doesn’t have a range of self, the spell’s range increases by 100 feet. If the spell has a range of touch, its range becomes 30 feet.

Empowered (Uncommon). When you roll damage for the spell, you can reroll up to three damage dice. You must use the new rolls.

Extended (Uncommon). If the spell has a duration of 1 minute or longer, double the spell’s duration, to a maximum duration of 24 hours.

Heightened (Rare). Choose one creature affected by the spell. That creature has disadvantage on the first saving throw it makes against the spell.

Quickened (Rare). You change the spell’s casting time to 1 bonus action for this casting.

Subtle (Common). You cast the spell without any somatic or verbal components for this casting.