If we go full Platonic I think circles should be shaped as circles. Circles were quite important and significant shapes for the classics, and Hermetic Theory was built upon that philosophy. And it should be an easy and limited target anyway, as the cost in magnitudes is the same of individual, not requiring additional magnitudes (I know they are still limited because you have to draw the circle and that takes time, but still).

That stonemason spell seems like an exploit to me, getting two magnitudes down to a spell that for me would involve a T: Part for modifying part of the ground, and probably at least one extra magnitude to get some flexibility in the target shape.

I thought there was a virtue that let you have square and triangle circles. Maybe the math-mancy hedge tradition?

How much earth could he be possibly destroying that this is at all necessary?

Base 3 Destroy earth, +1 touch, +1 Concentration, +0 ind

so as a no-roll spontaneous spell you can be constantly destroying 10 paces of soil that you touch. This would probably be interpreted as a rough type of destruction, not minecraft style dirt mining. But after that you rough in the sides. The whole process would only take about as long as it takes to inscribe the circle in the first place.

In short, if you are looking for increased efficiency digging the foundation should fall pretty far down on your pareto.

There is a Mystery Cult (or possibly just a Cult Mystery) that gets a boost from their ability to make geometrically perfect circles. Ergo, ordinary magic circles are not perfect. They're in Mystery Cults, I believe.

At the same time, we have the Columbae with their Minor Virtue which includes

Which seems to imply the normal Circle can't really be truly any closed, continuous shape.

The core book uses the term "ring" for the shape, which would seem to imply rounded roughly like a circle.

I take all this to mean it doesn't have to be a true circle, but at least rounded to be shaped somewhat like a circle. This keeps both of those Virtues' circles distinct from the shapes of normal Hermetic circles.

So a normal 'circle' or 'ring' is roughly circular, but can encompass kinda-oval-circle and coughed-while-drawing-circle? But does not include squares, triangles, squiggly lines designed to avoid a corner.

That's how I would play it based off the originally designated "ring" description and the perfection of said ring to get a true circle or alteration of said ring to allow odd shapes allowed by the two mentioned Virtues.

You might go with the TMRE specification of "rough approximations to a circle."

I'd also include geometric shapes more circular than a square - ie, a pentagon or greater. That's just personal preference based off of aesthetics, though.

And yeah, the hermetic Geometry (minor) virtue grants bonuses for perfect circles, as well as allowing any circle/ring spells to be cast ceremonially, AND grants bonuses for using additional geometric shapes in your ceremonies. (Mysteries, pg. 94). It's actually rather better than I thought it was when I first read it, as it stacks on itself. The other benefits (line, arcane circle/ring, etc.) seem kinda fluffy, but the bonuses above seem totally worth it for a warding specialist.

personally I'd want it to be at least a novagon (9 sides) as inscriptions with smaller numbers of sides are frequently used for other purposes (I think we can consider the Globe theater, with 13 sides, to be effectively a circle...)

I wouldn't.
'Corners' of any kind breaks the circle, or at least the concept of the circle.

So, the mystical importance of the circle is that it is an unbroken "line" that comes back to connect with itself. Thus it becomes the symbol of unity and the value of '1'. This is how the circle and everything inside it becomes a symbolic individual.
Now, I'm aware that you can draw eg a square with a single, unbrokenline in the sense that you never take your pen off the paper/chalk off the floor, but that's not the point. It still has corners, which have the symbolic value of lines crossing and thus of multiplicity.

At least that's my understanding. So no, I wouldn't even call the 13-sided Globe to be sufficiently circular.

the text on p. 94 of the Mysteries implies that a triangle can be used for ring/circle spells...
the problem with "no corners" is that corners become very oblique with a large number of sides, and simple error in writing can blur the distinction between a circle and say a 365 sided polygon. Its a nice argument and theory, but the text doesn't really support it and it breaks down if considered ad absurdum- simply walking with a 2 legged gait will introduce enough error the create some kind of corner.
Now intent may play some role in this as well (I was trying to draw a circle!) 13 sides makes each corner a 152 degree angle (where continuing straight is 180), and if you have seen pictures of the globe theater it looks very close to circular- though obviously something made with wood planks cannot be perfectly circular...

Not really. Let's quote the text we are all referring to:

This does not mean that any closed and continuous curve counts as a circle.
It means that any closed and continuous curve that is also roughly circular counts as a circle.

You've mistaken what that sentence is referencing. Notice how that appears in a box about using Hermetic Geometry and follows the switch to "Hermetic Geometry grants bonuses (see Ceremonial Figures) for regular shapes..." Look at the Ceremonial Figures part of Hermetic Geometry. The rules for magi without Hermetic Geometry (or other such Virtues) are that the shape needs to be a "ring" and that "so long as it is closed and continuous, and the magus successfully traces the line while casting the spell, even rough approximations to a circle count" as such rings.

I have question about the topic:
What happens if I create somethink with ring duration and individual target parameters? For example human corpses. Is it possible? With creo magic I mean. Is the object last until it is moved outside of the ring?