I do this with quite a few spells, which allows spells in the corebook to be used as is, but restricts the ability of Magi to modify them. I am of the same mind as other who believe Ignem needs a near monopoly on damage spells to be relevant and allowing this spell to scale as-is pretty much kills that. Our saga is pretty harsh on combat spells (Magi are scholars not warriors!), bumping pretty much every damage guideline up a magnitude or else requiring it to be aimed (and still have to penetrate!). Creo Ignem for instance usually has to succeed on an attack, but we judge spells as being pretty point and click and so usually give them a +5 attack bonus like crossbows. This serves to make most Magi less effective in at just killing people, but can serve to increase a dedicated combat Magus's damage with the attack advantage.
To me the Rego requisite implies that a Finesse roll to hit should be required and, since there isn't, it's probably a major benefit from the experimentation table, worth at least a magnitude. The spell says it speeds off like an arrow which makes me think the damage value should be somewhere around 5 (javelin) to 8 (longbow arrow), so the fact that it does +10 damage means that the inventor took his already lucky spell and got another side benefit; if we assume a crystal dart is similar to a javelin in damage, that would be worth a magnitude.
So I would say that the level would be roughly 10+damage desired, which would make it 5 levels higher than a similar Ignem spell as well as require a requisite, which I think is fair. Of course, I am probably over-conservative when it comes to "combat spells".
On the other hand, what if you were to create an enchantment quiver that shrunk down crystal objects placed inside, say a sharp crystal the length of a spear but 3-4 inches in diameter that became the size of an arrow? Then use a Rego spell to fire it. By the rules it would be a pretty low level spell without Muto needed (5? 10 if it was decided it needed a diameter duration?) and could be used in such a way that bypasses magic resistance (with 4 pawns a vis and a ritual to create the crystal of course, though if your Herbam is good you could just have them made mundanely).
On the other, other hand, it seems that the damage of the spell comes from the force and mass of the material. Why does it create a 10 inch dart? Crystal is the equivalent of stone, so why not a cubic meter ballista bolt? That should work without increasing the level of the spell, though I might have (probably) overlooked something.
Overall I would go with my first suggestion, keeping it weaker than Ignem by a level. If you supply your own bolts you can drop the requisite, or rather, the requisite becomes the only technique needed. You could also get the same affect with CrTe(Re) and not have to worry about being near dirt. My justification would be that to accurately control the missile requires higher levels of Rego as it gets larger or faster (i.e. increases damage).
Definitely. If everyone wants the same spell, it's obvious that it is a great spell. Which is fine, some spells are "better" than others. The problem as I see it is if those better spells are allowed to be made even better or at least easily. If a Magus gets a good experimentation roll and shares his lab text, great. You can't just take that lab text and change the spell though, that would be the equivalent of saying every positive side effect is a very minor breakthrough.
If you accept that the Crystal Dart is a result of experimentation then you need to decide how that would interact with other Magi wanting to change it up. In our Saga, where my Magus almost always experiments, we don't allow someone who learns a modified spell from a labtext to change the spell in anyway that couldn't be done with the normal guidelines. The inventor himself however can reinvent the spell again with experimentation and possibly modify the spell with another side effect, good or bad. He may also attempt to invent the spell again, but create the effect +/- one magnitude. He must experiment but as long as he doesn't ruin the experiment, he gets the change, and quite possibly more depending on the experimentation results.
My 2 pence anyway.
I seem to remember that Incantation of Lightning ignores armor, but I can't find that anywhere now. Is this just a house rule we have or is does lightning ignore armor?