Tied with Grass, Rock types have the most weaknesses of any type, with five. Additionally, double weaknesses among Rock types are very common.
Since Generation IV, the Special Defense of a Rock-type Pokémon is increased by 50% during a sandstorm, in addition to being immune to the damage caused by it.
Rock-type Pokémon, on average, have the lowest Speed of all Pokémon and of fully evolved Pokémon.
In particular, Rock pairs well with Ground, since very few Pokémon naturally resist this combination. Most Rock-types can learn Earthquake to this end. The biggest drawback of Rock-type moves is the large percentage of moves which have less than 100% accuracy.
Generation III introduced the most Rock-type Pokémon of any Generation, with 12, and Generation VII introduced the fewest Rock-type Pokémon, with five.
Generation IV introduced the most Rock-type moves of any Generation, with six, and Generation VI introduced the fewest Rock-type moves, with only one.
In the first season of the anime and during much of Generation I, the Rock type was often wrongly assumed to be unaffected by Electric-type attacks. This was likely due to the fact that at the time, most Rock-type Pokémon (namely the Geodude and Rhyhorn evolution lines, and Onix) were part Ground-type and thus immune to Electric-type moves. However, the only non-Ground Rock-type Pokémon around at the time (the then five Fossil Pokémon) were actually weak to them.
The Rock type is tied with the Grass type for the largest number of weaknesses when unpaired, with five. However, a combination of the two types (Rock/Grass) would only have four weaknesses.
Rock moves are super effective against four of the five types Grass is weak to, with Poison being the exception.
Every known Fossil Pokémon is part or solely Rock-type. This is most likely due to the placement of real fossils in rock formations, as well as the process by which fossils are created.