Ruby of the War Mage

Etched with eldritch runes, this 1-inch-diameter
ruby allows you to use a simple or martial weapon
as a spellcasting focus for your spells. For this
property to work, you must attach the ruby to the
weapon by pressing the ruby against it for at least
10 minutes. Thereafter, the ruby can't be
removed unless you detach it as an action
or the weapon is destroyed.