Heatstone

Source Inner Sea World Guide

Alchemically treated to enhance their natural heat-generating properties, these round or ovoid stones of volcanic glass provide enough heat to keep chambers warm in the coldest winter. One heatstone keeps a 20-foot-square area comfortably warm even in extreme cold (below -20° F), or a 40-foot-square area in severe cold (between -20° F and 0° F). A single heatstone is activated by striking it against any hard surface, after which point it continues to provide heat for 24 hours. An active heatstone does not give off enough heat to cook food or cause damage.