HearthStone Green Mountain Wood Stove. Clean Traditional Cast Iron. Our GM40 wood stove is styled with clean lines and offered in traditional black cast iron. Built for the wood stove purist. TruHybrid Reliability Our Green Mountain stoves exceed EPA clean burning standards. You can be sure they will burn clean today and for many years to come. Soapstone Firebox Lining.The GM40 is lined with soapstone blocks. Soapstone absorbs the heat of the fire to give you long-lasting comfortable heat.