Yellow-green to green-yellow gem-quality haüyne was found for the first time in an alkaline plutonic rock from the active Oldoinyo Lengai volcano in northern Tanzania. The mineral has optical and physical properties that are typical for sodalite-group minerals. Infrared and Raman spectroscopy showed that in addition to the (SO₄)^2- group, the haüyne also contains (NO₃)^-, (CO₃)^2-, and water in its structure. SEM-EDS and EDXRF analyses confirmed the presence of Na, Ca, Al, and Si as major cations, with minor amounts of K.