Stones and crystals are used for three major purposes in the Sylvan Tradition. While they are also used for other purposes, such as in healing work, these three areas are where they find their most regular use. (The stones I refer to here are small enough to be easily carried and are generally not large enough to mark the boundaries of sacred space).
Magical "batteries": Much like magical tools (see Crafting Ritual Tools for charging techniques) stones can be charged with magical energy and that energy be used in ritual work. Once charged, simply hold the stone in your receiving hand during the point of the ritual where you project energy. As you pull the energy from inside yourself, from the Elements around you, directing it toward your goal, you can also draw it from the stored energy within the stone.
Magical properties: Different stones have different properties. Tigers eye aids in perception (both physcially and spiritually), quartz crystal is a meditation aid, hemetite is used for grounding, etc. An extensive listing of properties accompanies many books on the Craft. To use the stone for that purpose, simply hold it in your recieving hand (like you were going to use it as a magical "battery"), but instead of drawing from the stone and projecting the energy outward, simply draw those properties within yourself and use them.
Witness effect: While I don't have the space to cover this concept in any sort of detail here, I'll briefly touch on the concept. Time, as those of us who work magic know, is non-linear. As such, when we work magic in a site that has been used for countless rituals, we can tap into the combined energy from every rite that was ever performed there. This "memory" of a site is simply known as "the witness effect." Stones may be used in the same manner. For that reason, I have a stone that I have kept with me in almost every ritual since I began my path. If needed, I can use it to connect with the sum total of energy from every ritual the stone has "witnessed."