Not actual rock, jd116.
It's just the word astronomers use to describe the mixture of MOSTLY dust with SOME ice's.
at 80 odd percent the comet nucleus is actually MOSTLY empty voids.
So, for want of a better word and thanks to Reality Check we can now use the word ROCK.
but we understand not actual ROCK but some majical mixture of NOTHING, DUST with some ice.
hardening of the sub-surface layer
Comets: looking ahead Michael F. A’Hearn
The Rosetta mission orbiter Science overview – the comet phase M.G.G.T. Taylor (1), N. Altobelli (2), B. J. Buratti (3) and M. Choukroun
but not actual rock.