Weight: ½ lb.
Estimated Value (Sane Cost Guide): 500 gp
DMG Value: 101 gp - 500 gp
A mind crystal contains sorcerous potential, allowing a user to apply Metamagic to a spell as you cast it. For the non-sorcerers among us, Metamagic allows you to tweak and alter a spell to better suit your needs, such as extending its duration or increasing its range. While Metamagic is a sorcerer class feature that needs sorcery points to use, mind crystals are single-use items that can be used by anyone who can cast a spell.
An extended Mind Crystal doubles the duration of a spell that already has a duration of 1 minute or longer, up to a maximum of 24 hours. Encounters rarely last longer than 30 seconds, so these will only be beneficial for combat spells if your party is moving quickly from room to room in a dungeon. For this reason, an extended mind crystal is often best suited for utility or exploration spells, or spells you might use on a heist.
Spell suggestions: charm person, death ward, disguise self, enhance ability, invisibility, mage armor, pass without trace, polymorph, suggestion
Source: Phandelver and Below: The Shattered Obelisk