Is is such a thing as 'pure matter' and 'pure energy', or is
everything always a combination of energy and matter? EG: light being
mostly energy and some matter, and rocks being mostly matter but some
energy, for instance?

Hi david

The answer to your question depends on 
answers to several subquestions, like

What is energy?
what is matter?
Are these two different physical quantities?
and few more (like does it matter :-) and what is 'pure' ?

These people 
http://www.learner.org/channel/workshops/energy/workshop1/
and most elementary textbooks say:

           "Energy is the ability to do work"

 I do not like this definition. It begs question:
 what is work?  (applied energy?)
 
 and it is wrong as demonstrated by following thought experiment:
 
 --------------------\\
  
  Imagine a rigid thermos [made of steel ..] (a box which will not allow any energy
  (thermal, mechanical, [magnetic ..]) pass it's walls.
  
  In the thermos there is flashlight, left on. Battery has (chemical)
  energy, which can run a small DC motor and so could 'do work' --
lift marbles or so..
  But - box is closed and after few days battery cannot power the
motor and 'do work'.
  Battery is 'dead' and chemical energy of battery was dissipated into heat,
  There is is still same amount of energy in the thermos,
  but that energy cannot 'do work'. That ability was lost. 
  
  ----------------------------//
  
   So that definition is useless.
   So - what is energy and what is 'pure energy'?
  
  One way is to look at these EVOLVING concepts is historical. 
  
  Other way to interpret the question in terms of current
  elementary particle theory.

  e.g. :  there are photons - particles of light - which are hard to
          put into a test-tube (less impossible, lately) but hard.
	
 and there are things made of atoms,like  water and air, which go well
into test tube.

BTW : ------------\\
Unless you keep these materials (made of atoms) at absolute zero
(which is impossible to achieve)
http://www.madsci.org/posts/archives/dec96/835000890.Ph.r.html
materials always have some (heat) energy.
------------------//
 These (atoms) are  ultimately made of neutrons and electrons and protons
 and more ultimately of leptons and quarks. 
 
 So that is one way to answer your question is
 (light is pure energy) even when it has 
 'relativistic mass' - it has no rest mass.
 There are other similar particles.
 
 Generally - that classification is here:	

There are particles and forces - brief history
http://www.ph.ed.ac.uk/particle/Theory/slides/rrr.pdf

the proton and neutron - examples of baryons - are made of quarks.

Fermions and Bosons
http://www.shef.ac.uk/physics/teaching/phy304/fandb.html

---------------------------

 There is another issue here which has to do with relativity
 
 Are these two (energy and mass) different physical quantities?
 
 People keep quoting thus mantra E=Mc*c, we are even going to have 
 an MC square in Princeton, with Einstein's statue,
 
 but I really wonder what does it mean to them. It does not mean that
 nuclear fission or fusion is possible; There are some historical connections,
 and they did use that when figuring the fission experiments, but it really is not
 about nuclear energy. It is more general than that. So what does it mean?
 Can someone elaborate?
 Hedgie

Hedgie grappled with my question from several perspectives -
philosophical and physcial - and provided several useful links. In all
this I found the answer I was looking for. Thanks very much.

E=mc^2

pure energy can be found as the resultant of a fusion reaction where
the mass of the new particle does not equal the mass of the two
previous particles and the difference is converted into PURE ENERGY

Toni 23...thanks.

E=MC2....which I understand as meaning energy *interchanges* with
matter at the speed of light squared. The formula does not *itself*
tell me that there is 'pure energy' or 'pure matter'...does it?

I can't put a piece of gravity into a test tube; gravity only exists
in the context of matter. And the fact that more matter = more
gravity-energy (more gravity on big earth; less on smaller moon)
evidences this...doesnt it?

But I **really** don't know what I am talking about. 

gregorious....thanks too. 

That is helpful and i think I might even understand it. I see from
what you say that we can *infer* the existance of 'pure energy' in
this reaction. However, could it be that that 'pure energy' is just a
couple of 'strings' - comprised of energy and matter - that went off
to do their thing, possibly, so leaving us with a smaller particle? Or
do we know better?

Also, where does this leave 'pure matter'? 

thanks again.

In order for a mass to have no energy it must be at absolute zero,
which as of yet is unobtainable.

Thank you m2cahueg-ga...that was the main learning from the answer I
was provided. I note - with considerable interest - your caveat:
'absolute zero is *as yet* unobtainable."

Still leaves the questioin of 'pure energy'. Should absolute zero be
obtained one day, then presumably 'pure energy' would be a neccessary
by-product, given the Law of Conservation of Energy.

Thanks for your interest. 

David