And another definition..

Bob,
Here's the definition(s) we give our first years (you'll find something
similiar in most first year university level Geology texts)

MINERAL
A naturally occurring solid possessing a characteristic chemical composition
(or range of compositions) and a systematic 3-dimensional atomic
(crystallographic) structure.

 e.g. quartz SiO2
  pyrite  FeS2
  calcite CaCO3
  olivine (Mg,Fe)2SiO4
(the last example explains why the "or range of compositions" bit is in the
definition - it accommodates the solid solution found in many minerals)

CRYSTAL
Any mineral grain bounded by natural planar surfaces (i.e. crystal faces.)

ROCK
A naturally occurring, coherent, multigranular aggregate of one or more
minerals.

 e.g. granite, sandstone

Quite clearly, petrified wood, fossils and coal are all rocks rather than
minerals.
Hope this helps,
cheers
Bill


"Bob" <[EMAIL PROTECTED]> wrote in message
news:[EMAIL PROTECTED]
> I am stumbling to find a true definition of a mineral, to help me
> figure out if the following are indeed considered minerals:  A bone -
> A shell from a crustacean, Petrifed wood, a fossil, and Coal.  Can
> anyone help out there - thanks - Bob