As of now it seems like you can't get gems and mounts from heroic boxes.
Gems like:
https://www.wowhead.com/tbc/item=305...ious-fire-opal
https://www.wowhead.com/tbc/item=305...rial-tanzanite
They have like 3-6% drop chance from each boss according to wowhead. Now tbf, i dont know for a fact if they drop from bosses on Onyxia as im not playing there, but they should and thus im asking you to please add them to HC boxes on blackrock as well.
And then there are mounts:
https://www.wowhead.com/tbc/item=327...the-raven-lord from sethekk halls
https://www.wowhead.com/tbc/item=304...arhorses-reins from atuument the huntsman in Karazhan
Let them be really low drop chance but please add them :)