Re: BPM 37093 'lucy' is a diamond?

You are quite right to think there must be more to this news story than it
first appears!  Diamond, as you know, is composed of a crystal lattice of
Carbon atoms.  The white dwarf in question, BPM 37093, is also thought to
have a crystalline structure, and may well be made predominantly of Carbon.
However, as you've realised, the density of the white dwarf is much much
greater than any material found on Earth, and so calling it a giant diamond
is taking a bit of liberty with language.

That said, the true nature of white dwarves is every bit as fascinating and
exotic.  When a star ceases nuclear reactions in its core, it collapses
under its own intense gravity, and forms a superdense plasma in which the
atoms are literally crushed together so they overlap, and further collapse
is only prevented by so-called degeneracy pressure of the electrons.  The
foce producing degeneracy pressure is a consequence of quantum mechanics,
and in particular the Pauli Exclusion Principle which states that no two
electrons can occupy the same quantum state -- which means they can't be
compressed arbitrarily close together. The theory suggests that is it cools
 this substance is likely to develop a crystaline structure

So, how do we know that BPM 37093 is such a crystal?  In reality this
discovery was the main point of the news story about this object.  The key
to this breakthrough is that BPM 37093 is a pulsating variable star: it
undergoes rapid, regular vibrations, alternately expanding and contracting.
Just as earthquakes send sound waves through the centre of the Earth
which have been used to deduce the structure of its core, so pulsations in
stars can give us a picture of their internal make up.  In this case, the
large crystaline core produces an identifiable signature in the spectrum of
vibrations of the star.