After some banter in a game of Ars Magica RPG I’ve been corrected in my understanding of how Creo magic works. Till now I thought that Creo spells never required a Finesse skill check to cast, unless they were creating a very specific item to the caster’s desire.
Particularly I thought any natural item will always be fine, and if that Finesse check passes for an unnatural thing the item should be fit for purpose (with the natural vs unnatural thing being very generous). I thought the Finesse roll was almost always required by Rego spells which emulate crafting activity, because the Rego activity is constructing the item rather than conjuring it fully formed. Thus I was wrong that Creo only really needed Finesse rolls for style, appearance, or presentation of an item.
The core rules state that Creo effects must use a Finesse check if the thing conjured is artificial. Further if the Finesse check botches then the item is not fit-for-purpose. As a guideline it does not provide much more than that.
(the Theory of Forms only has room for natural things? A shame to miss the humble chair)