This gem has 3 charges. As an action, you can speak the gem’s command word and 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 ships the D&D 2014 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.