I want to make sure I understand the RAW correctly regarding Spell Target and Size.
The sidebar of the main rules on page 113 "Targets and Sizes" (2nd paragraph) seems to basically say: for every Size +1 above the Form base size, add a magnitude to the spell level. Is that correct?
No.
For each 10 times the mass of the base Individual, add one magnitude.
Page 192 explains that 3 points of size is a change of about a factor 10 in mass, meaning 3 "sizes" means 1 magnitude.
It should be noted for corpus, a base individual is anyone up to size +1, and then +1 magnitude for size is anyone up to size +4, by that.
Thanks for the clarification!
Anyone fancy doing a table showing magnitude added sizes for Terram as a square, cube and sphere? 
Do you need it?
It's just square roots and cube roots really.
Not everyone is a mathematician, Tellus.
I don't now but for a lot of players it would save time to have such a chart
Strictly speaking, neither am I.
I'll try to remember about them when this darn headache leaves me.
I whipped up something quick and dirty. I didn't check my work, but everything seems about right. If someone notes an error, it's easily fixed. I know I could have done a better job with the formulae cascading properly from cell to cell...
docs.google.com/spreadsheets/d/ ... sp=sharing
Feel free to copy the spreadsheet to your own account, I just shared it for viewing.
The first group covers the base size for all the terram types. The second group has a spot for adjusting the size magnitude to get the volumes. I made meters=paces=3 feet for simplicity sake.
Thank you that is really useful! adding 3 magnitudes to base can make you one huge gem it seems!
Well, it was wrong. 
I had done the work in Excel and copied and pasted over. I thought I caught all the formulas, but those for adjusting the size magnitudes didn't get replaced. I just updated them. So, a gem of Size +3 would only have a spherical radius of 6.21 inches. Not sure if that's a good deal for 8 pawns of Terram...
Right, well done.
I won't worry about this then.
Let's see...
6.21 inches = 15.8 cm radius.
So it gives a volume of 16'438 cubic cm, which is about 16 liters. From a purely esthetical view point, since a spherical diamond has no facet, it is pretty boring, although it has interesting optical properties and can be a nice focus for Imaginem or Scrying activities (but you can do so much better with 8 pawns of virtus IMHO). It is the cut that reveal its fire and brilliance.
If you consider that diamond has a density of 3.5 (on top of my head), that's a spherical diamond of 57.5 kg.
As a rule of thumb, white diamond of the best quality and cut will fetch around 10'000-15'000 USD for one carat. One carat is 0.2 g. The spherical diamond is 287'000 carats. I let you do the remaining math and I am not going into the consideration that the bigger the stone, the higher the price per carat.
The issue is not if it is worth it, but how you are going to sell it ? who can afford it ? You will need a good ReTe with excellent Finesse total to cut your bowling diamond into something more salable. Something like 500 diamonds of 287 carats each (considering the cutting loss of about 50% - even with magic - but you can recover the loss to make many smaller diamonds).
The largest cut diamond is the Cullinan I, with a weight of 530 cts, coming from a 3'000 cts rough stone.
And if we want to really go into details, talking about cut, it is only at the turn of the 20th century that the modern cut that reveal the full beauty of a diamond was discovered (based on optical properties, refractive index, light path) and finetune recently with the help of computer. So to have a real nice cut diamond, you not only need a high finesse, but a good understanding of geometry and optics (probably covered by Artes Liberales). In the Medieval time, they did not have the tools, nor the knowledge to do anything better than just basic facetting using cleavage angle. So diamonds were usually the shape of an octaedron (d8), with on tip cut to make the table (the flat. horizontal surface where the light comes from).
Of course, all that is relative to modern time, so the Mythic Medieval paradygm is something entirely different. You can probably trade your mammoth diamond for lands, with a castle and some villages, with the hereditary title of Lord of the Sparkling Orb.
Inflation is a terrible thing, in older time, with some luck, you could find a king willing to trade "My kingdom for a plate of lentils", nowadays a mammoth diamond might just be worth a few villages...
In medieval times, gems were more often left uncut, merely polished.
Except that diamond can only be polished by diamond power, or shaped by wearing it down against another diamond.
However, it was possible to do a rough facetting process by using its cleavage (weakness of the stone at specific angles), hence the octaedron shape.
Finally, in Persia, Egypt and India, facetting was done on many stones. It was discovered a long time ago that various materials had different hardness and could be used to shape other softer material.
It was crude, and you would never get the shape we are used to see, yet it was possible.
My commentary about the worth of eight pawns (really nine, because I forgot to include + 1 magnitude for Touch) to create a diamond with a radius of 6.2 inches was vague, but meant to highlight the difficult of dealing with such a creation. I don't think that Mythic Europe would be able to handle such a large gem, well.
All this, and more. Of course, one could use the same 45th level spell to make 100 "standard" sized diamonds. Group +2, Size +1 (applied to Group). A Terram master would certainly know that he is capable of creating such an object, but there has to be a purpose behind it. Certainly such a large gem falls in the Size x3 multiplier on the shape and material table, and it would be a priceless gem, meaning 20 base points, or 60 pawns of vim to open it. Such a gem is useless to a magus...
... who doesn't have Verditius Elder Runes.
...where Magic Theory * Philosophiae=60.
Magic Theory 10, Philosophae 6. Entirely do-able for a verditius.