Amorphous diamond is coal. ;-)
Not quite
. Diamond is carbon with 4 covalent bonds per atom (the
shortest and strongest chemical bonds known) with a nominal cubic
structure, graphite is carbon with 3 covalent bonds per atom and a
nominal hexagonal structure. Either of these materials can be found in
amorphous, polycrystalline of single crystal forms.
When thinking of amorphous materials like glass there is a tendency to
think of them as a distinct state of matter; but if you look close
enough, on the scale of a few atoms, you will see the same sort of bonds
as in crystalline forms of the same substance, but more strained on
average. If an amorphous material is heated near melting, the scale of
crystalline order will slowly grow from a few atoms in an ordered group
to large, distinct crystals; clearly polycrystalline, but there is no
distinct phase change involved, just a continuum of increasing scale of
order and decreasing average bond strain.
Neither amorphous nor polycrystalline properly describe coal, it is
somewhere in between, but it is almost entirely graphite type bonds
rather than diamond bonds. When the scale of order of these graphite
bonds is so small and randomly ordered that material properties are
essentially isotropic, the material is generally referred to as amorphous
carbon. Coal can vary widely in it's scale of order, from glass-like
through large graphite flakes. Most engineered carbon materials are
derived from petroleum or natural gas, not coal, due to the inconsistency
and high level of impurities found in coal.
