"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 special 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 own spirit. Through these, the power of monk's mind is made manifest, the reality itself being altered through the bearer's sixth sense.