As I read it you focus your Shards spell on any item of wood or stone within your presence (Origin), gouging massive splinters from said object and flinging them at great velocity into your enemies within paces of the item (Area of Effect). The spell leaves deep grooves in the material used as its source.
If I am reading this correctly then my mage should be able to cast Shards on a bow or even an arrow that is being knocked and drawn as the source material for the spell, not only to attack the bowman, but to weaken his weapon or obliterate his arrow as well (causing deep grooves in a bow is bad, causing them in an arrow is worse).
It also would seem that causing deep grooves in any material would weaken it a step or two (cast Shards often enough on the same support beam and you get a cave in).
I doubt that this was the original intent of the spells use, but I can see it as a creative use of it.
You need a big chunk of material to spray shares out of. A bow is not gonna cut it. And while you leave obvious grooves, any material thick enough to provide good shards is thick enough that the damage is just cosmetic.
Does my mage need to be able to see both his attacker and the source material for his Shards spell? For example, what if your target is crouching behind a rock or tree? You saw them take cover, and you can see your side of the stone or wood material item that they are using for cover (and obviously within a pace of) Can your spell still be cast against them by affecting the side of the cover item thsts towards them or do you have to have clear line of sight to what your targeting and what you are targeting them with?
Shards is not about targeting materials, it’s about gesturing and hurling shards of stuff at someone. I honestly don’t think the wizard has any choice in what makes the shards, nor is it something of importance except cosmetics and vanishingly rare cases with no wood or stone around where maybe he couldn’t cast the spell. But the spell at base is just a damaging spell with shredding as flavor.
While I agree that you can’t really use Shards to compromise the structural integrity of an object, you can certainly use the cosmetic effect – i.e., the deep grooves on the object – as a bluffing tool. Cast shards on a stone floor, and then tell someone that you’ll do that to the more delicate wooden supports unless they meet your demands (Sorcery --> Intimidation linked test). Or, cast Shards on a support beam, and convince people that they need to evacuate before it collapses (Sorcery --> Falsehood linked test).
I think its also fair to say that Shards could do damage or destroy certain objects in certain cases. For example, I would be willing to say that by casting Shards at someone on the other side of a wooden picket fence would blow a hole in the fence. In general, any scenario where the material used for Shards is a large collection of smaller objects should result in destruction or serious damage to the objects. However objects this small probably aren’t being used for structural support.
In general, I would be wary of players using Shards in a way to grant tangible, relevant effects other than damage. I.e., no blowing open doors or destroying shields with Shards. Destroying a fence is fine because its not terribly important. I would almost certianly Say Yes to breaking it down anyways.
There are a fair number of things that you can destroy by casting Shards at them. Chunks of stone can make holes in all kinds of things! And I can see gesturing and having chunks of something rip off, hover momentarily, and the violently smash into whatever they came from and make a bigger hole. But you have to cast Shards at the thing to be demolished, not just near it.
So you couldn’t destroy a cross beam by using it for your shards spell, but you might be able to destroy it if it is the target of your spell (perhaps using the very stone above). This would depend upon the rules for breaking things then wouldn’t it?.
I can recall an instance from a distant campaign where our wizard used Shards to destroy a crossbow. It was a great moment, but purely up to the GM if it was possible. And that’s probably where I’d keep it - in the hands of the GM to decide.
I think it could be used with intent and task in a versus test like that but not in a fight! where things are happening way too fast.
But as you say it is the domain of the g.m. to decide such things (and I’ll add, possibly a group vote).