What is the drop rate/acquisition rate of these and are there rules tied to it like the fight MUST yield xp (can’t be full team of 25s to farm) or things like that? I’ve never seen one of these drop and there doesn’t seem to be a lot of information on these.
From numbers from at least Rosqo and me, and I think Disco back in the day, the rate of drop is about 1%. At least, that’s what I’ve always assumed. And any of the Blue or Yellow stones can drop - that is, there is 1% chance for a stone to drop. but the stone could be any of them.
But I have no solid back up for this. It’s just based on anecdata.
If one had dropped for you, would you have noticed it? or thought nothing of seeing a stone in yoiur bags?
Definitely can drop from the full PvP queue, since AFAIR, that’s where Disco got his stash of them when he was levelling a character entiorely in Gilneas theough the PvP queue. I’m pretty sure that was Rosqo’s evidence too. I would have got mine from farming and levelling.
i’ve done hundreds of pet battles (probably thousands in total from way back) and have never seen one drop 
but most of my pet battles are max level lv 25 fights to either catch pets in new rarities or just try and see if a stone drops. i feel like i’d definitely notice since I want to find them to sell at least; i haven’t seen any stone of any kind drop whether race-locked or unlocked.
I’ve seen marked flawless and charms (extremely low quantity) drop from random battles, but it is very, very rare. I don’t think it’s even 1%, but have no data. I just remember when I was leveling some pets on fight night in Dalaran last week and a stone dropped and I had to stop for a minute to try and figure out what just happened. 
lol
So, yeah, rare. They really should up the rate. Especially for charms. Boost it to 10% for a 1d5 charm drop, which is almost nothing, but it would make pvp fights and daily quests more interesting. Stones should be at a 2% drop rate, I think.
Honestly, they wouldn’t be breaking the bank if they dropped at that rate. It wouldn’t affect anything but our interest in the activity.
That’s my 2 cents, anyway.
I had one drop this weekend and it made me think how it’s been years since I had seen one. I thought they had been taken out of the game. I’m a pretty active battler too, across a lot of characters. Then I had another one drop today. I was actually coming to see if anyone else had suddenly started seeing these again, like there had been a bug for a while or something and it’s been fixed.
Maybe I’ve just been super unlucky.
I just got one recently too and I only do Pet Battles in TWW zone now…I just checked my bank alts bags and the flawless one is in later bag so that means I got it just lately …I don’t remember if I have had any others but I do have 1 right now.