Could be some have a lot of sulfur in them.
"Oxygen and sulfur are the third and tenth most abundant elements in the solar system. The isotope ratios of these elements are different whether found on Earth or a meteorite.
Enough nonsense!
All of the comets are of Earth origin.
They were ejected during the Genesis 6 events. Kepler’s theorem proves that this has to e true, since they are co-planar with only the Earth and the sun, not the other planets.
They are mostly water and silicon dioxide, but do contain trace amounts of other elements.
No comet could have rained fire on those cities.