My friend gave me very good herb tea, all natural, in tea bags. It’s made from Stivia leaves. I am not sure - do all teas need to have a KOSHER sign?

All teas have to be Kosher even if they have natural ingredients, the natural ingredients may be non Kosher, some teas have oils in them. Therefore, unless one knows that the tea has been accepted by a reputable Kosher authority as Kosher - one should not rely merely on the ingredient list.