Scorching Obsidian Staffstaff, legendary (requires attunement)

The shaft of this staff is made from a petrified branch, worked to almost a mirror finish. It is topped with a large chunk of obsidian that comes to a twisted point. This staff has 5 charges and regains 1d4+1 expended charges each day at dawn. While attuned to this staff you can tell how long ago volcanic rock was formed, and what type it is.
As an action you can expend a charge from this staff and aim it at a point on the ground or a wall within 60 feet. When you do so the obsidian glows white-hot and a pinpoint ray of immense heat fires from the staff. Each creature within 5 feet of the ray, or 15 feet of the target point, must make a DC 16 Dexterity saving throw. A creature takes 4d6 fire damage and 4d6 radiant damage on a failed save, or half as much damage on a successful one. If an area affected by this ray is composed of sand or stone it too becomes white-hot before turning to a black glass over the course of a minute, and any creature that starts its turn touching the area during that time takes 2d6 fire damage.