This gem has 3 charges. As a Magic action, you can expend 1 charge. For the next 10 minutes, you have Truesight out to 120 feet when you peer through the gem.
The gem regains 1d3 expended charges daily at dawn.
RollQuest summary 3 charges; regains 1d3 at dawn. Magic action, 1 charge: Truesight 120 ft for 10 min when peering through the gem.
RollQuest ships the D&D 2024 SRD as a content pack, on a 5e character sheet that does the maths - roll from the sheet and let the numbers work themselves out. Free to start.