As you gaze at fire, you give witness to this ongoing interplay that produces combustion. The dancing flames show evidence of burning gases. Notice that nowhere is the combustible material, be it wood, coal or paper, aflame. It may glow red which is a sign of its heated state and conversion to gases. Flame displays its lively nature as it stretches out engulfing unburned gases. In the process the quantity of combustible material reduces. It has been converted to burnable gases; its unburnable component is ash. Some is light enough to be lifted away on the upward flowing air currents. Heavier ash sinks to a small pile left after the fire has extinguished.
For all material, this is the fire process. The difference is what temperature is necessary to drive off combustible gases. Gases that are combustible at normal air temperature only need spark. These gases are hazardous to handle. Even liquid gasoline does not burn . However, it’s temperature to convert to burnable vapors is lower than temperatures found outdoors on a mild day. Then spark is all that is needed to begin the inferno. Other burnable materials have combustion temperatures in the range produced in ordinary fires. Other materials will not combust in these fires. Examples are most metals, ceramics, stone, and rock.
But even these things can burn in conditions well beyond the ordinary. Some have combustion temperatures quite high. Even metal can turn to vapors at temperatures high enough. These temperatures are not normal and only found in controlled environments like science labs, deep furnaces, or nuclear experiments. At times, certain other elements need to be present to catalyze burning. They conditions may also occur in the bowels off the earth where high temperatures and pressure produce fires that have powers of transmutation. The ashes from these fires are known as gems and sometimes called precious.