The Cooling of White Dwarfs

A white dwarf shines not by nuclear reactions (unless it is in a nova phase), but by radiating thermal energy.

Schematics of White Dwarf Cooling

Remember Stefan-Boltzmann: L ~ R²T⁴. So a hot white dwarf has a high luminosity...
...but Luminosity represents an energy loss rate: L=dE/dT...
and the energy of the white dwarf is thermal: E ~ NkT

So in the beginning, the WD is hot -- it radiates lots of energy (its relatively bright) and cools rapidly. As it cools, it radiates energy more slowly, and is less luminous. The evolution would look something like this:

But they don't cool like this forever. As they cool, the nuclei in the WD lose their thermal motions and settle into an ordered crystalline lattice. This process releases heat, slowing the cooling for a while. But eventually lattice vibrations cool the WD even faster.

Ultimate end state: A crystallized sphere of cold dark carbon/oxygen.

Question: What's another name for crystallized carbon?