Concrete is often made up of sand and cement. The sand is often silicon
dioxide, which is already oxidized. Some sands are made up at least
partly of calcium carbonate, which also has oxygen in it. Calcium
carbonate is not a good material to include in concrete because it is
water soluble and would wash away.
Most stones are oxides of some material or other (most often
silicon, calcium, boron, aluminum, magnesium, phosphorus, potassium,
manganese, and lots of other materials). In this sense, they are
already "burnt" as well. Many rocks are formed in volcanic eruptions
and begin their lives as hot lava. If they weren't already combusted at
that point, then they certainly would catch fire.
In fact, most combustible stuff is living or has been living at
some time in the past. Wood, oils, fats, gasoline, and natural gas all
receive their energy from the sun as transformed by living organisms.
Some metals burn (like iron -- very fine steel wool can be ignited, as
can pure magnesium, and sodium and calcium and cesium will all explode
in contact with water), but they are not found in their pure state in
nature (otherwise they would have burned long ago).
(republished on 07/13/06)