It's made from petroleum refining and natural gas processing.
Propane is normally a gas, but it's stored in liquid form on a vehicle.
Once the propane enters the engine, it becomes a gas again, which helps this fuel to burn so cleanly. The propane fuel grade used in vehicles is called HD-5. Propane is widely available. Hawaii has over 300 refueling sites throughout the islands. Propane fuel tanks are 20 times more puncture resistant than conventional gasoline tanks.