I've seen a 59 monk using BC heroic gems:
https://worldofwarcraft.com/en-us/character/bleeding-hollow/It
He's since switched to the int/crit PvP vendor gems, but preceding the nerf to resilience he was using
Steady Chrysoprase in all of his gem slots. He's so far the only 59 (or, for that matter, any character under 70) I've ever seen using BC heroic gems, so I'm guessing the method used to obtain them on a sub-70 is either a really well kept secret, or they might possibly be grandfathered from some earlier time when loot eligibility rules might have been different.