"The magic of any gem is dependent upon the magic of the light that gives it life and fire. Gems are complex things and handle light in complex ways. Light doesn’t just uneventfully flow through windows as it does through glass, or simple bounce back as from a black-hearted mirror. Instead it dances impatiently, refracts and reflects. It comes alive along with the gem. In a weird way, a gem is a crystal cage that traps the light and makes it fight to escape."
—Diane Morgan, Fire and Blood; Rubies in Myth, Magic, and History