Considering that the trainer was a Psychic, I'm going to assume it's a metaphor as a compliment.
Using google, rockhead means
- A guy who's addicted to cocaine
- A dumb guy
- A guy who isn't flexible in Japanese (Credit: Qwerty_Zoom)
- Upper surface of bedrock and rock salt.
Since a rock isn't shiny becoming shiny like a gem probably means you became better person than before, which means stopping being whichever of the above
I highly doubt he means the first one, and if it's the second one then it would be offensive even if it is compliment coming from a psychic. The third one makes sense since I believe flexibility is an important trait for a person to have for a psychic.