The subject says it all: are pearls Animal or Terram? We know that they are "animal products" and therefore Animal, but I wonder how the ancients saw them.
My two cents:
Pearls, even though they start as a grain of sand inside an oyster, take years for the oyster to create from that speck of dirt. Therefore I would say pearls are Animal, since without the oyster, you cannot get a pearl.
My guess is that the ancients would have seen them as a stone, since it is not uncommon for some animals to eat stones to aid in digestion (like chickens). But I do not think that this should effect what spells effect pearls. Even though we try to play within the Medieval Paradigm, I personally do not think what people believe should affect what things truely are. Unless you allow dillusional magi to believe people are actually trees and can be controlled by Herbam and not Corpus, for example.
A similar argument could be made for coral. Just because it looks and acts like a rock, doesn't mean that it is a rock.
Mother-of-pearl is a product of an animal.
I thought animal products were things people made from animals, like leather, bowstrings, pigskins and such. You know, things that used to be a living part of a creature. What is a pearl made of? Calcium carbonate in crystalline form. That itself wasn't alive. So I would argue that, even though the pearl was created by an animal, and sat inside an animal for its whole existence, it would still be made of stuff classified as terram.
If venom is Animal and not Aquam because it is produced by animals, I think pearl should be Animal. No ?
Exactly why we never go with the "they thought it worked like this so it does" thing, because it ultimately creates precisely the problem above. And its much easier to run with REAL physics regardless how ingame characters THINK it works.
Its actually a rather problematic example i would say.
It most certainly isnt directly animal, or part of the animal, but at the same time it is created by an animal, although of course you can argue that a stone statue doesnt become Co because a human carved it...
I would say it gets treated as Te... But thats very nonabsolute even as i write it...
But the poison is directly and intentionally created within the animal, a pearl is incidental due to layers of material forming around some base material that ended up in it.
It comes from inside an oyster and though it is hard, it is still from the animal. I would put it in the same category as animal bones: Animal.
IIRC the description about Terram says that it delas with all forms of earth as well as other hard substances if they are not clearly something else(or was this 3rd edition?), due to this I would support Terram as well.
Just looked it up:"in addition to ... can sometimes affect inanimate objects in general." and "This Form concerns solids, especially earth and stone", personally I feel this is slightly better than considering the pearl as part of the animal since there are a lot of oysters without pearls.
Maybe treat it similar to a gemstone as an unusual form of Terram.
CharonJr
And that is the question, and the only one that matters.
I have not posted here in a while, and it worries me that I return to find that some seem to be happy to toss out opinions based on their own, quite modern, understanding of pearls. All this arrogant nonsense about calcium - as if someone somehow knew how pearls were made! pffft. Can you say "Medieval Paradigm"? Sound familiar?
DW muddies the waters - the ME is not what "persons" believe, certainly not what characters understand, but what was commonly understood to be "how it worked" - one madman, moron or confused soul does not change the physical consistency of the world. The "reality" of the ME in Ars' is not mutable - dragons are real because that's what "people" understood - they do not cease to exist in an isolated town where no one individual person believes in them. So with any detail less well studied by the common "person".
"We" are not authorities, nor is any character in a saga - only history is. and the understanding of the world at the time.
Speaking of which, less than 1 minute googling gave me this:
http://www.farlang.com/gemstones/kunz_book_of_the_pearl/page_058
Circa mid-1100's, a learned scholar explains...
"... on the 4th and 20th day of the month Tisan*, a certain dew falleth into the waters, which being sucked in by the oysters; afterwards, about the middle of the month Tisri*, men descend to the bottom of the sea, and, by the help of cords, these men bringing up the oysters in great quantities from thence, open and take out of them the pearls."
(* - Hebrew calendar)This "dew (or rain) to pearls" theory goes back as far as Pliny (Historia Naturalis)
http://bestiary.ca/beasts/beast548.htm
"...The fruit of these shell fishes are the Pearles, better or worse, great or small, depending on the quantitie and qualitie of the dew which they received... If they may have their full time and season to feed, the Pearles likewise will thrive and grow bigge; but if in the time it chance to lighten, then they close their shells together, and for want of nourishment are kept hungrie and fasting, and so the pearles keepe at a stay and prosper not accordingly..."This continued to the mid-1600's, without anyone really being able to explain "how". Dew was taken in, and the pearls got harvested later - how does it work? Just fine, thanks for asking.
But it's clear that the oyster did, indeed, somehow, create it, an Animal process rather than a mineral one. Some sources claimed that the pearl was soft until removed from the water, at which time it hardened - maybe it's Aquam?
For me, I would say Animal, all the way - Pliny himself says, "the fruit of the oyster". No different than a lion's tooth, a bear's claw, or the horn of a rhino or unicorn - tho' the nature of Terram could perhaps move it around, and Aquam could possibly better speak to its origins and so forth - not every item needs to be wholly and exclusively of one Form.
However, all that said, this is one of those questions that may have no "right" answer, or several. Any SG worth their salt will ponder the situation, and make a decision, perhaps after consultation with the troupe, perhaps not, and with an eye to what works best in their Saga. The only wrong decision would be to not be consistent in your reasoning - decide what criteria need be considered, and stick to it.
Salvete Sodales!
I'd vote for Animal for the reasons mentioned above, as other things produced within or by animals (e.g. silk) seem to fall under this form. This does not neccesarily mean that any magus knows this, but if they try to manipulate a pearl with a Te-spell they might be in for the same surprise that other get when casting He-spells on silk.
Vale,
Alexios ex Miscellanea
Except that that is complete bullshit, because the "medieval paradigm" isnt an all powerful "commonly understood", its the parts that have been selected as the medieval paradigm.
Which means it IS totally arbitrary.
Why thank you for so graciously accepting that oh allmighty and all knowing one. 
We had kinda already figured that out in case you missed it while ranting about our dreadful shortcomings in adhering to a makebelief reality based on selected parts of what MAY have been known the greater part of a millenia ago.
And of course you KNOW that those "explanations" were the ones the people who actually dealt with it had as "common knowledge"? And that it was not a matter of a philosophical explanation, like how many Chinese scholars treated the earth as flat, on a philosophical basis despite the fact that they still knew that it was not.
We also know in modern times that faeries don't exist, demons aren't real and hermetic magic doesn't work.
The core of the game is a medieval europe where the beliefs and knowledge of the world at that time are accurate and true. Faeries do exist. demons do cause disease as do imbalance of humors.
We do have to try to put a little modern science in to try to fit the hermetic theory but pearls were of animals by belief of the time. It would be animal as the bones, teeth, shells, and horns were animal.
Animal, of course. Also the "ancients" knew pearls are from animals.
Speak for yourself. I happen to believe demons are real.
Ok, I'm convinced, storyguides will probably say that pearls are Animal. But you have to agree that it isn't obvious.
A magus finds a pearl on the beach. He is kind of sheltered and doesn't know what it is. He tries to affect it with a Terram spell. It is, in fact, completely made of non-organic materials, unlike bones or horns, which have cellular structure and components that used to be alive. However, little does the magus know, the pearl has such a strong arcane association with the oyster/clam that made it, and such a strong cultural relationship with the oyster/clam in the society of the time, that it itself must be Animal. The spell fails. Great.
By your logic and oyster shell, found on the beach, could only be effected by terrum spells since it is just made of largely calcium carbonate. Ignoring the fact that an animal made it.
Bezoar-stones would be another example.
Okay, pretend I am Albertus Magnus, the most brilliant scientist of that era.
Explain to me what "non-organic materials" means, as well as this "cellular structure" thing of which you speak.
Hey, I gave up already, but i guess I'm still grouchy 
. And I guess it doesn't matter what exactly CaCo3 counts as in modern science... I'm pretty sure it's inorganic, but that's a little blurry, since it contains calcium - what I meant by organic was alive or formerly alive. As far as medieval paradigm goes, I would tell Albertus Magnus that non-organic means it was never a living part of a living thing, and if you examined its tiny parts, it would look more like a crystal than a bone. I unfortunately don't know the exact terminology that Hermetic Magi would use.
I do kind of ignore the fact that an animal made the pearl. The clam got a piece of rock or whatever, and cultivated it until it was a bigger rock. Consider the parallel situation where a seed is planted in the ground. The ground supplies some Terram and Aquam, and later the plant absorbs some Auram and Ignem (sunlight), and grows into a plant. Is the plant Herbam, because it started as a seed and it definitively a plant, and therefore Herbam... or is it some combination of Terram, Aquam, Auram, and Ignem because these are the things which created it and cultivated it? If you ignore the elements that created the plant, why not ignore the animal that created the rock?
I think Animal related pearls gives more flavor to the game.
And following the logic kidney and gallstones must be Corpus. 
I m curious : what do your magus want to do by casting spells at pearls ?