Ruby

"Fatis Draconis Incendium!"
You can expend a Ki point while holding
this stone to call forth a raging fireball
from the stone.
All creatures within a 15 ft. radius must make
Dexterity saving throws. On a failed save,
those within the area take (2d6/Ki point)
points of fire damage,
and half as much on a successful save.
This includes the monk
at the center of the explosion