By definition a rock is a collision with an object.
If the object is the road, or affixed to the road, it falls under collision (or if it's another vehicle, as well).
If the object is flying, it's comprehensive.
Here is BC's insurance:
https://icbc.com/insurance/products-...n-Optional.pdf
“collision coverage” means coverage for loss or damage caused by upset of a
vehicle or collision of a vehicle with another object, including, but not limited to,
(a) the surface of the ground, the roadway being travelled on or an object on, in,
under, over or adjacent to the roadway, including a road sign, guard rail, pier,
bridge or culvert or any body of water or waterway under or adjacent to the
pier, bridge, culvert or roadway,
(b) a pedestrian as defi ned in Part 3 of the Motor Vehicle Act,
(c) a vehicle attached to the vehicle, and
(d) cargo, including animals, carried in or on a commercial motor vehicle, the gross
vehicle weight of which exceeds 5,000 kg, or a commercial trailer,
and includes coverage for loss or damage caused by collision with another object
where the collision results from the presence on or adjacent to the roadway of
a domestic or wild animal, either living or dead, but there is no impact with the
animal;
“
comprehensive coverage” means coverage for loss or damage other than loss or
damage to which collision coverage applies and includes coverage for loss or
damage caused by missiles, falling or flying objects, [...]