Your question is not for me but:
Creating pearls? Or magic items, talismans. 
"Bullshit"?! Is that the scope of your vocabulary, or does it reflect your usual reaction when someone disputes your position? Or are you just going for a forum ban? Either way, try to stick to discussion, or debate if you disagree, and not sophomoric sputtering.
And that does not, for a second, mean it's "arbitrary", any more than any recreation of history is, but it does require that one sit down and consider the What's and the Why's - which is what some of us are doing here. My criticism was that some of the rationales presented above were modern ones, and needlessly so - I got to the same place, coincidentally, using the ME. If you'd rather use modern rationales, great, enjoy your game, but don't tell me (or anyone else?) to change - I'll stick to the game as presented, because even if the details of the ME are slightly different for each troupe, they will be consistent within my Saga. If all that's too much to think about, then don't, but don't attack those who can wrap their minds around it, please.
Are you suggesting that we cannot "know" any history as fact? We cannot "know" about the Norman Conquest or the Mongol Invasion, we can only read about it in books. And yet, I feel that I know what happened in each instance, or close enough.
But, yes, at that level, I "know" that explanation was the dominant one, if not the only. And that "knowledge" will last until I am presented with something to the contrary that is at least as convincing.
And, again - the ME does not have to be "common knowledge" of all people of the time, but more for anyone who believed they could explain how it worked. Quite often "the learned" were quite confused about animals they had never personally encountered, while the rural folk knew "the truth" from experience - up to each Story Guide which way they want to go on that.
Sure, why not?
The easiest way to deal with questions like this is not to over-think it or "explain" it, but simply to be dogmatic about it - magic does not have to be completely "logical" at an intuitive level. It's magic, it's beyond mere mortal understanding. 10 Forms, don't make it too complicated. If it comes from an animal, it's Animal - if from a human form (humans, some fae, some others), it's Corpus. The fact that a pearl is considered a "gem" in the vernacular should not confuse anyone for a second that it's Terram.
We all have preconceptions of what a drinking cup is made from, or a dagger - clay, or metal, or glass - Terram. But a cup made from a rhino's horn or a dagger of ivory would certainly be Animal, and not Terram.
Interesting effects and responses can be achieved by making items from non-standard materials - when Animal does not work on the parchment scroll, see how long it takes the wily magus to realize it's made from human skin, or fae,. Think outside the box, and then see if your players can as well.
(B - I can see a magus being very frustrated that his InTe "Find the Treasure" spell fails utterly when seeking pearls.)
Jeez. This is just a game. The rules of the way things work can diverge from our own science, and from reason. So there is no point to applying reason if we're trying to come up with an answer to a mystery about the game. What is necessary is for the ars magica writers, or the storyguides for the individual games, to step in and say, "it's animal," or "it's terram," or "surprise, it's actually vim!" The game's not so perfect that all the answers can be derived from what they wrote about their world, so whatever you think you can conclude that isn't given to you in black and white is probably just your own interpretation.
This is fun to think about, but there's no reason to get emotional. I personally have cast off my grumpiness. And I don't really care about pearls anymore.