Janet! Dr Scott? Janet! Brad? Rocky!

Dirty snowball, boulders! Dust, boulders, icy dirtball? Dust, large boulders, rocky? Cliffs, snowy dirtballs, fragmentation of boulders! Small boulders, dust, rocky iceball? Weathering of boulders, rock falls from cliffs, deep fried ice cream, ice boulders? Over 3000 large boulders found so far on comet 67p's surface! Boulder fields, icy small Solar System body? Sublimating ice boulders? Rocky!

If it looks like a duck, swims like a duck, and quacks like a duck, then it probably is a duck.
Duck test | wikipedia

Selected quotes below from an ESA (European Space Agency ) comet 67P article called ... A fall of comet dust and a field of boulders.

From the earliest clear views of comet 67P/Churyumov-Gerasimenko it was obvious that the surface is a collection of contrasts: smooth plains, imposing craggy cliffs and scatterings of boulders.
comet 67P/Churyumov-Gerasimenko boulders rocks geomorphology cliffs
Dusty plains are not the only thing covering the surface of the comet. There are also boulders scattered across the dramatic landscape. Maurizio Pajola, University of Padova, Italy, and colleagues have identified 3546 boulders larger than 7 metres in size.

On an imaged surface of 36.4 km^2 – this is 75% of the total surface of 67P – the number of boulders larger than 7 metres averages to about 100/km^2.
A fall of comet dust and a field of boulders | ESA

Sublimating ice boulders

where what how comet 67p boulders rocks come from origin

These boulders are not uniformly distributed across the comet. On the smaller lobe, often dubbed the head, there are more smaller boulders than on the larger lobe, called the body.

In the neck region, there is more uniformity between the number of large and small boulders. One particularly intriguing boulder field (see anaglyph above) occurs here and the team suggests that it was created by blocks falling from the cliffs of Hathor, which are located on the body of the comet.

The team has identified three possible reasons why there are fewer small boulders in this field. It could be firstly, because small icy boulders have sublimed away; secondly, small icy boulders could have been ejected from the comet’s surface by outgassing events; thirdly, airfall could have covered the smallest boulders removing them from view. It could be any one or all three mechanisms at work simultaneously.
A fall of comet dust and a field of boulders | ESA

Boulder weathering

Planetary geomorphology is the study and comparison of solar system objects that uses Earth as the basis. Amazingly to geology theory it seems that no matter what material the comets, asteroids, moons and planets are made of the geological features have similar features. Including on the water based comet 67P.
comet 67p boulders rocks weathering fields erosion

In particular, the size distributions are related to how fractured the formation area is. Thermal stress causes fractures on the surface, which can dislodge blocks - forming new boulders - and there is also a continuous fragmentation of boulders that have already been formed. By examining the frequency with which boulders greater than 7 metres are found on the head, the neck and the body of the comet, the team shows that the head is more fractured than the body: the size-distribution for the head is steeper than that of the body.
A fall of comet dust and a field of boulders | ESA

These fractured boulders and rocks look similar to rocks found on planet Earth, Mars and many other planets. Even the debris field around them is very similar to Martian and Earth rock fields.
comet 67p Boulder field in Hapi region rocks rocky origin geology
Using planetary geomorphology scientists have even had to discover what can only be (according to geology theory) water erosion features on asteroid Vesta.

And they now have sublimating non rocky rocky looking ice rocks to add to geology mysteries. Oh Rocky!