There was more facts from the creator himself in Spanish, or more specifically where I found the James Sr. twit:
I couldn't exactly figure out what the first fact was, but I can translate the rest:
1) "...direct contact turns them to stone - this is traditional and is in all legends."
2) "The amulet admits different stones, but is only an amulet (created by Merlin) using a secret metal- which is revealed in chapter 3"
Okay, obviously this "metal" Merlin used to create the amulet was Morgana's left hand. So, I wonder if the reason why these stones accept and upgrade the amulet is because the amulet's metal is made from the flesh of another powerful wizard like Morgana.
3) "Vendel is dead- if they turn to stone, there is no remedy if they BREAK THEM. Only Merlin and Morgana can repair them after that."
Obviously, it's a confirmation that Vendel is killed off for real: if a troll's stone body is shattered, there is no way to reverse the petrification. Only powerful wizards (like Merlin and Morgana) have the power to repair a troll's stone bodies. This was shown when Morgana used her magic to resurrect Angor Rot, although Merlin was never shown doing this.
4) "Jim's dad is a secret until the final season of WIZARDS!!! Shhh-"