There have been several space probes that have visited comets, and I have never read anything that would support the hypothesis that they might be composed of diamonds.
Also remember that in the EU paradigm, a comet could be composed of anything (some stars could be described as "cometary," the term could possibly be used with regard to certain galaxies). "Comet" describes a celestial objects' electrical status not its' composition, but most within our solar system seem to be rocky bodies; asteroids on elliptical orbits which puts them under electrical stress. Mainstream seems to be sticking with the dirty snowball or icy dirt ball theory, or a loose conglomeration of ice and rocks or what have you. Most here think that the days for that theory are numbered since visits by probes have falsified that theory.
As far as the diamond planet is concerned, there is a thread here