AV>>> a way to deploy a diamond coating on things which would increase
  >>> the speaker efficiency a lot (imagine... diamond-hard cones. Btw,
  >>> more clarity than ever
  >DH> I would imagine that a speaker cone with a diamond coating would
  >DH> fracture the first time you applied a signal to it...
AV>But remember that the cone is supposed to stay that way: conical. Else, we
  >would be using rubber cones!.
But we *are* using rubber cones!  
When we apply any force to any structure, it will flex.  Sometimes we
cannot detect that flexure but it happens.
AV>DH> To make a speaker cone work better, it should be totally rigid.
AV>So much the better to apply a diamond coating over it.
I still have a hard time with that.  Any structure will flex.  For a
speaker, you will want to have something that is rigid and light weight.
Adding a thin coating of a highly brittle substance over another
structure is not going to make that initial structure any more rigid
unless that second coating comprises a major part of the total assembly.
There *are* some major uses for the thin-film diamond coating but they
are totally centered on the advantages which diamond offers. They are
for low wear and temperature conduction.  I have not seen any commercial
use of diamond for resistance to bending.
Imagine taking a sheet of cardboard and bonding a thin coat of glass
to it.  Take the cardboard and bend it - the glass will crack.  This is
a no-brainer!  This is the same thing that will happen to any diamond
coating on any speaker cone.
AV>DH> Placing a diamond coating on a speaker cone sounds like a wonderful
  >DH> exercise in marketing hype if the speakers cost less than $1,000US
  >DH> each.
AV>Not so sure... diamond coating is achieved not with diamonds, but letting
  >diamond develop over the surface to be applied.
I have read about the process - it is done by putting the object to be
coated into a vacuum chamber and depositing carbon onto it.  At some
specific mix of temperature and vacuum and gases, a diamond will form.
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