Analysis of Types: Chapter 12: Rock-type: Part 3: In-Universe Symbolism and Mythology

So, what is Rock-type's role, really?

The Legendaries and Their Role:

Golem:

Regirock:

Regirock is a generic rock golem. That's really it. It is generally speculated that it represents the stone age, just like how Regice is speculated to represent ice age, and Registeel is speculated to represent iron age. Regirock's symbols are just braille that doesn't mean anything, and its body is composed of rocks from many places because it constantly fixes itself.

Mountain-based:

Terrakion:

Terrakion is based off Porthos from The Three Musketeers. Its section is Trial Chamber and its Species is Cavern Pokémon. It lives in canyon regions to test others' and its own capabilities.

Glass-like:

Nihilego:

Nihilego is based off a jellyfish, either aurelia aurita or Phyllorhiza punctata. Its' parasitic behavior might be based off Myxozoa. It's crystal-like structure and its neurotoxin's addictive nature seems to be based off a personification of crystal meth and other inorganic psychoactive materials.

The Trainers and Their Roles:

Brock:

"The Rock-Solid Pokémon Trainer!" Brock is the first Gym Leader and one of the archetypal Gym trainers in the game. Though he claims he trains Rock-types due to his rock-hard determination, in reality it is due to the fact that Pewter Museum exists near his Gym.In later games he was retconned into a Breeder due to his animé appearance.

Roxanne:

"The Rock-Loving Honors Student" she is a Pokémon School graduate who wants to use her expertise in real world as a Gym Leader. In ORAS, she was retconned as a Fossil Pokémon expert who writes studies about Fossil Pokémon in science journals.

Roark:

"Call me Roark the Rock!" Roark is a miner who took over his father Byron's gym though Byron thought Riley first for the position. Due to this, Roark seems to have developed some small inferiority complex. His Fossil Pokémon are fossils he dug from Oreburgh Mine.

Grant:

"The wild, yet cool and collected, rock climber." Grant is a triathlete who predominately bikes and rock climbs. Due to his cool attitude and status as a professional sportsman he is popular among young people.

Olivia:

Island Kahuna of Akala Island, she keeps a jewelry store that also imports fossils from other regions. Her store is the only way to obtain them.

The Overview:

In terms of Legendaries Rock-type is truly dry. The few species that exist have Rock-type as a description, not as a theme. Their theme is generally more based on mountains, except Nihilego, who is just esoteric(pun intended).

In terms of Rock-type trainers, it seems that most of them have some association with Fossils, including Grant, than other Rock groups, with Roark and Olivia being actually involved in mining, and Roxanne is known to go for Rock hunts with Steven(we presume Brock does the same). Two of the recent Rock-type trainers are fairly fashionable despite keeping their cool. In fact, most of the Rock-type trainers except Roark seem to have a strong will and commitment, and some level of unintentional introversion. 

While Rock-type is abundant in terms of type experts, it is not too popular among Legendaries, and until that changes Rock-type's thematic status will remain a bit buried.