when creating realia, what are the criteria for objects to be added? For example, if making a collection of artwork for Imagonem what would be the minimum Artistic Quality? I would guess 9, but it could be higher, would it be dependant on the level of realia (perhaps necessitating replacing some of the collection as it improves)?
What sort of items would you have in a Vim based realia? Artifacts of pre-Christian religions?
Can a realia be assembled by someone ungifted? I would think so- after all anybody can make a collection of objects of interest and significance, but that certainly allows grogs a much greater supporting role to magic...

I would go higher than a 9 Aesthetic Quality. That only requires Characteristic +3 plus Ability 4 w/ specialty while taking time (+1). I would expect talented artist to have Free Expression, Puissant Ability, or Great Characteristic along with Ability 6. So I would probably target 12 or so.

As for Vim, I would allow unrefined Things of Virtue as well as the things that naturally store vis (such as parts of beings where the vis is found). Not limiting to that, but saying I think all those should apply well.

I would not allow anyone to deliberately assemble a realia for any subject they could not write a book for. they might manage to do so accidentally, perhaps; A king gathering a collection of legendary weapons might accidentally make a realia for perdo based on the methods of killing, or perhaps some less silly example.

Would someone with an interest in philosophae (geology) be able to assemble a realia that would serve for learning Terram? An artist with magic theory be able to create a realia for imagonem? By the description a realia is made up of mundane things. I can see your point about a book, but with the nature of realia I would think close should be able to manage, perhaps if being advised by someone who does have the ability/art in question...
also can realia be used for more than one topic? Stone statuary for both Terram and imagonem? A collection of weapons for both single weapon and Perdo...?

Consider silveroak's artwork example. It doesn't take the same skill to assemble the collection as it does to create the works of art. Similarly, it doesn't take nearly the same understanding to copy a book as it does to author one. There is no need to be able to wield any of the Arts to copy books about them perfectly. But Magic Theory is needed. I would side with silveroak (How often do we agree? Heh!) on this one. I would probably place the same Magic Theory requirement on the collector as on the copier to make sure the pieces they gather are appropriate, like making sure the copier understands the text enough to avoid making mistakes even without knowing the Art.

Perhaps a Vim-based realia collection could be assembled out of objects previously used as Aegis tokens? There's adventure/story potential there - new covenants represent a chance to expand the collection, searches through redcap records to find out what former covenants used for tokens and exploring their ruins, etc.

could a library be considered a mentem realia collection? One game's library is over 1000 books... Level 10 me realia? Or only books over a certain Source quality (or aesthetic quality for entertainment books...)

Long after the Ars Magica period, there were cabinets of curiosities, and I think that's what Covenants is trying to go for, as well -- you're studying from a collection of interesting things, not a book.

So the library itself probably can't be a realia collection, although it could by its nature produce vim vis per season or be used for a study bonus (for those with appropriate virtue or flaw) to intellego or vim.

I'd also say that objects in a realia are often found (at least in their original state), not made: e.g., interesting stones, taxidermied animals, objects that gained realm alignment. I do think a magical realia collection could be assembled by an ungifted character with sufficient Magic Theory, although the quality may not be as good (unless he/she also has Second Sight or another way of intuiting magical value).

This is a link to a thread from last November which has a ton of example realia.

It might be helpful.

perhaps just literature can count towards a mentem realia collection, to prevent a magical library from also being realia...

A skilled artist can make great art, even intentionally create a masterpiece, but it's difficult to deliberately create a timeless classic.

So too with realia. I can see where a mage might make an effort to make a magical exemplar, but achieving this is probably beyond intent.

Look at, for example, some of the efforts of literature. Mary Shelly, Bram Stoker, Herman Melville, all had one classic masterpiece and other works.

I don't know that pieces of Realia have to fit the definition of timeless masterpieces. After all these are things that can be picked up as merely "excellent specimens"