Mind Crystal (Subtle)
Wondrous Item, common
This gemstone contains a crystallized bit of spellcasting magic.
When you cast a spell that has a casting time of 1 action while holding the mind crystal, you can modify the spell in the way 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 the mind crystal, it becomes a nonmagical gem worth 50 gp.
Modify Spell
You cast the spell without any somatic or verbal components for this casting.
Source: Phandelver and Below: The Shattered Obelisk p. 218