Twinkle, twinkle, little star,
How I wonder what you are!
Up above the world so high,
Like a diamond in the sky!
Well maybe the title is a bit far fetched, but I’m really stoked to find out about such a thing; the star in case, BPM 37093 is a variable white dwarf star that consists entirely of crystallized carbon, in the form of diamonds (well, technically speaking, body-centered cubic lattice of carbon; basically, crystallized in the cubic system).
Previous research had indicated that as a white dwarf cools, it crystallizes, starting from the center, and more recent work shows that about 90% of its mass has already done so, making it technically the biggest diamond we know of, at about 10 billion trillion trillion carats.
Astrophysicists nicknamed the star Lucy, after The Beatles song Lucy in the sky with diamonds. Man, you just gotta love astrophysicists.