I used one stone to get crit on an item from 6 to 12.
But for a friend of mine it took 19!!! stones before it crit changed to 9 or so what it is now. Yesterday an other guy bought 16 agility stones to change his block rate (yes all from me so I was happy, although I sold it cheap)
So its all pure luck based. Just take your time, I use it mostly on SoS or better items (or those just before sos like 38/48/60 etc.)
Otherwise its not really worth it due to the fact that one of those stones costs quite some gold (or gold you could have earned).