Zombie villager was cured and changed to a mason. Immediately traded until mason was rank 2, and sold stone until unable to trade at a rate of 13 stone :1 emerald.

Waited for rank up to happen then sold diorite until he ranked up to master. 

Night passed. Trade was changed to a rate of 29 stone : 1 emerald. proceeded to trade to see if it would continue. I ran out of stone (I did not max out the trade). Logged in this morning and now the trade was at a rate of 41 stone : 1 emerald.

I understand there is supposed to be inflation on frequent trades, however I have never had nearly this much after far more trading of the same type.