Normal Gem is the only gem that has been discovered in the game. It is very likely that the other gems do not exist and will not exist given that Game Freak seems to be going for lower power on moves (nerfs to the the old 95 BP and 140 BP elemental staples).
Yeah, I had a suspicion that this would be the case this generation when I saw the BP dropped. Bit of a shame since I'm a fan of them, but the change does make things a bit more interesting.
I've seen conflicting information on the web about various methods such as the above mentioned rock smashing, but without any concrete evidence or follow-up.
Personal experience: I did try rock smashing at terminus cave for about ~40minutes - no gems found. Haven't tested at various times of the day.
I also tried that furfou thing for fun, but I never really expected anything out of it.
Cheers everyone for the responses.