Spirit Stones

"Spirit stones are used by the monks of Ivgorod to channel their energy and focus, allowing the power of their mind to seep forth."

- Game Guide

Spirit Stones are a specific type of Helm worn only by the Monk class in Diablo III. These are the focusing crystals and enchanted stones through which a trained monk can channel his or her spirit. Through these, the power of a monk's mind is made manifest, reality itself being altered through the bearer's sixth sense.

Diablo III
In addition to stats rolled by all other Helm types, Spirit Stones may roll maximum Spirit, Life per Spirit spent and Spirit regeneration.

Spirit Stones may be transmogrified into normal helms, but only a Monk can do the other way. As of Reaper of Souls expansion, Spirit Stones can even be dyed, altering the crystals' colors.