The guidelines in Societates are for natural stones, with only a initial push so they don't have to penetrate MR under those guidelines. As a bonus, this means that the spell is viable with a Touch range: all the magic takes place at the start. That right here explains why the base is one magnitude higher.
The Crystal Dart has a Muto requisite (or rather a Muto base with a Rego requisite, for one extra magnitude), creates a perfect projectile (worth some extra damage) rather than using a random stone, and does have to penetrate MR. So the extra damage for the same push comes from throwing a perfect projectile rather than a random stone.
So if you travel around with, say, flint spearheads, or crystal darts, and use the Invisible Sling of Vilano spell from Societates (which is level 10), you should do the same +10 damage as the Crystal Dart for the same level.
Since the base for CD is 3, that means the ReTe guideline that moves the projectile must be level 3 or less. A quick check in the core book shows "Control or move dirt in a very unnatural fashion" is level 3. That seems ok to me.
So the presumption is that if the level 3 ReTe moves the object at an appropriate velocity, then the damage comes from the projectile type. CD is creating an item that does 25% more damage than a long bow. And the ReTe guideline in Societates does 25% more damage than a sling stone. So if I wanted to hurtle a javelin head then the spell would be base 5 + 1 magnitude for metal and the damage would be 6-ish (5 * 1.25)?
Does that make sense?
Oh wait. Control/move dirt is level 3. So, for CD, should there not be a +1 magnitude for moving stone/crystal? Or am I not understanding requisites?
The Crystal Dart doesn't use a guideline to set the damage it does, the damage was set arbitrarily before thought was given to how guidelines set damage across Forms, and the damage is probably based intuitively on the spell level rather than on any hard and fast guideline.
The crystal is still earth "undeneath" so moving earth is sufficient. The +10 damage is a composite of a highly-lethal battle-head (perhaps +6 weapon damage?) plus the "attack advantage" provided by the Rego effect; there is no guideline to set it precisely.
The Sling of Vilano is based on a guideline that was set as a response to a gaming-need, but fails to take into account the effect of having combat-designed projectiles; the guidelines it is based on also don't really say how to extend them for greater force/weight, if this is indeed possible (I don't see why it shouldn't be). The base says how much damage is caused by a blunt moving object - unlike the CD, the damage does not include an "attack advantage", since the spell does not direct the projectile to the target.
In principle, using a more deadly projectile (such as a javelin head) instead of a pebble should increase damage - but there are no guidelines for this. In principle, a very good aiming roll might also increase damage, providing an "attack advantage" of sorts - but again, there are no rules for that. In both cases, as an SG, consider carefully what ruling on this would imply for the rest of the saga - the game will get more bogged down in calculations and magi may increase their damage considerably with such rulings.
ArM is an abstract game. The game rules can't really simulate reality, not even a "magical" reality. The spells work as their texts say they do. Anything beyond that, is a house rule. Consider carefully your house rules - aim for playability and fun, not necessarily realism.
I was not factoring in some kind of aiming bonus from the Rego. Taking that into account makes everything click for me.
I still am curious why the base is 3 and not 4, since I believe it is base 4 to move a stone unnaturally. ReTe 3 + 1 for stone. Am I not understanding requisites?
I am not sure I agree with that statement. If there were no guidelines then I agree the spells work as they say they do (like in Harnmaster, for example). But, by publishing spell guidelines, Ars is making an effort to establish a set of rules for designing spells. I am just trying to figure out where my understanding of the guidelines is off since I can't reconcile CD with the guidelines.
I think that the projectile is still earth underneath, so there is no need to affect stone/crystal. Just like you can affect a human changed into wolf form (through Hermetic magic, not the Heartbeast mystery) by using Corpus (IIRC).
But the guidelines aren't as comprehensive as you're thinking they are. The damage of The Crystal Dart was set by hand, not by guidelines. If you want to create alternative spells based on the same principles, you need to pick the damage by hand too. It's a house rule, since it isn't in the rules. And there are no rules for using the Societates guidelines for throwing darts either, nor for extending them to greater forces/damage. You have to wing it. The guidelines don't cover everything.