I have searched and only saw one topic touch on this, so I thought I'd see what the boards think...
My theory is that the skills that generate with a grimoire stone are the actions taken during the "grimoire chance" round.
For example, I was in a battle with a Ragelope, and during that battle, had two "grimoire chances," one with Raquna, and one with Simon. during those respective rounds, I had Raquna use Fortify, and Simon use Regenerate. After the battle, I received two grimoire stones, with those skills embedded in them.
In addition, I believe that the level of the skill that gets generated is equal to half the current level of the skill that is used (rounded up). In the above example, my Raquna used a level 5 Fortify, and Simon used a level 1 Regenerate. The grimoire stones that were produced had a level 3 Fortify, and level 1 Regenerate.
Now, this is really my only evidence so far. I plan to do more testing... unless this is old news, in which case... I'll just be grinding :P
However, you seem to agree that using a skill will increase the likelihood of that skill dropping in the stone. So... maybe not completely wrong?
I wish I had more data. I haven't been playing it, because I am trying to beat EOIV before Millenium Girl comes out (I had stopped right at the dragons and the 6th stratum when Animal Crossing came out...).
That's partly why I brought this to the boards. I figured there were a lot of people here that might be Grimoire grinding, ergo a huge data mine to tap.
my anecdotal evidence points me to currently believing that character skills for grimoire stones are randomly selected from your current build, but that enemy grimoire stones will only be created if the enemy used that skill (possibly on that turn, but at least at some point in the battle).
My minor evidence is that I was trying to get a grimoire stone with the mantis slash ability. I fought probably 15 of the things (while using the home base buff that increases grimoire chance) and never got it because in all those battles I was binding its arms. Once I thought about the fact that if I want the skill I need to let it use the skill, and deliberately let it hit me in the face with the scythe skill before finishing the battle, only then did I get a grimoire stone with that ability.
The Grimoire has already been deciphered in the JP community. The Grimoire contains enemy skills used THE TURN Grimoire chance happens. It also chooses RANDOMLY from any skill the character has, passive or active. Does not matter if you use it or not. On top of that, there seems to be a priority of Enemy move > Active > Passive
The level is random for the enemy. It is also random for the character skills, but it cannot exceed the character's skill's level.
So if you want an enemy move, you have to be lucky enough that they use the move on the turn the Grimoire chance pops up
Tried to start a blog... --> http://tdnshumi.blogspot.com/ Plans: Thoughts on Megami Tensei 4, Etrian Odyssey Untold