Heater stones should be of a material that is as heavy as possible, dark in colour, and large in size for them to be able to store as much heat energy as possible. Heater dealers sell stones that are purposely intended for use as heater stones and which come from well-known suppliers, and whose composition and suitability are guaranteed.
Stone material that is well-known and suitable for use in heaters includes: peridotite, olivine and olivine-dolerite. Heater stones must be arranged loosely to ensure there is good air circulation.