"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(src)
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.