So one thing that GURPS Magic lacks is a good-old Wall of Stone spell. Oh, sure, you can use Create Earth to create some dirt, then use Earth to Stone to turn it to rock.
But what about a single spell that skips straight to the rock?
Let's look at some costs.
Create Earth: 1 second to cast, costs 2 per cubic yard.
A 9' x 9' x 1' wall would be 3 cubic yards. 10' x 12' x 1' (most generic 10' Felltower tunnel dimensions are 10' x 12') would be a bit more than 4 cubic yards, but you could probably round it down and assume it's a little less than a foot thick. So those are 6 and 8 respectively.
Earth to Stone: 1 second to cast, costs 3 per cubic yard. Turning the above to stone would be 9 and 12 respectively.
Total time is 2 seconds, total cost is 6 + 9 = 15 (13 with skill 15 in both, 11 with skill 20 in both) or 8 + 12 = 20 (18, 16).
Fast, effective as a barrier, but pricey. It's also two spells to get off, and a foot of earth isn't necessarily a strong barrier against some forces.
So how to build a combination spell?
You could just make a "Create Stone" spell to simplify it, and because you might not always want a wall.
Making it take a little longer (say, 3 seconds instead of 2) and cost a little less (4 per cubic yard, instead of 2 + 3 per cubic yard) doesn't sound crazy as mutual tradeoffs. Neither does eliminating the "metal" option from Earth to Stone. You essentially combine the steps, spend a little longer at it, and get a cheaper wall. I'd make it Very Hard just because it's a combination of two useful effects.
Create Stone (VH)
As Create Earth, but creates stone instead.
Duration: 24 hours (DF limits creation to 24 hours)
Time to Cast: 3 seconds
Cost: 4 per cubic yard, minimum 8.
Prerequisites: Magery 2, Create Earth, and Earth to Stone.
Or you can go for a more limited, literally Wall-type spell.
Wall of Stone
Creates a 1' thick slab of stone up to 4 yards tall (excess is lost if blocked by a ceiling, roof, etc.) around the area. Must be created in contact with the ground.
Duration: 1 minute
Time to Cast: 2 seconds
Cost: 4, same to maintain.
That's more like a "circle of stone." But it's shorter.
If you wanted a really flat wall, you could look to the 3e Grimoire and Force Wall and Utter Wall for ideas. Both are per-hex costed and 2/3 cost of the dome version. That would suggest cost 2-3 per hex, say 2, for a Wall of Stone that lasts 10 minutes.
Any thoughts on this? I'm not sure I'll use it, I'm just thinking about allowing what's becoming a regular tactic (create earth, turn it to stone) into dedicated spell for people who want to hurry the process along. I'm not really sure I'm satisfied about the costs and details. But I kind of stall out with those, so I figured I'd post them and let people see what I was thinking up until now.