Hearthstone

From the White Wolf Wiki, the most complete White Wolf reference

A hearthstone is an egg-sized crystal of pure solidified magical energy accreted at the center of a manse. If an Exalt attunes to the manse whence the hearthstone comes and carries the stone on his person in a focal device constructed from one of the five magical materials, he can draw upon the capped demesne's power to perform miraculous feats. The power and nature of these feats depends on the aspect of the demesne. The precise manifestation of power, however, is based on the geomantic Essence-focusing of the manse's occult architecture.