The act of casting Stone Spikes is magic, but the damage caused by Stone Spikes is damage caused by terrain, which is not a magic source.
More specifically: the Terrain described on Stone Spikes says "occupant takes 6 points of Piercing damage." The game runs by this text description. The card has it's own keywords, "Magic Piercing," which only apply to casting the spell as VNSAramaki says, and the spell itself deals no damage. Some might argue, then, that the Stone Spikes card is being redundant and weird by saying "Magic Piercing": the "Piercing" keyword will never be used during casting. I'd say this is an odd detail in how Terrain-creating cards work and is very likely to cause more confusion.
But Shimmering Aura does protect against Burning damage, so it seems the difference is between terrain attachments and char attachments. And I think it should be more consistent.
Oh ho ho! This is absolutely true! Does Shimmering Aura also defend against the Fire damage of a Hot Spot? I think it does. And this distinction matters because Shimmering Aura defends against Magic and Projectile Attacks, so eliminating "Projectile" would be helpful to avoid confounding variables.
Correct, any terrain damage loses the magical damage source - just as stone spikes, this says it does "fire" damage in its textbox instead of Magic Fire as per the cast. Cast is magical, damage source is non-magical.
You could easily argue the reverse though: When right-clicking to view the terrain attachment, you see the card. On the type line of the card, "Magic" is there. The terrain attachment is the card attached to the terrain. The card is magic, so the damage should be magic.
I agree that it's causing some confusion - however, the damage from the card doesn't come from the cast itself, but from the text in the text box, that specifies a non-magic origin.
Does Shimmering Aura really protect from Burning damage? Because if it does then isn't there an inconsistency here? The Burning keyword says "Attach to target. At the start of each turn, target takes <x> points of Fire damage." This is similar to the wording of Hot Spot/Wall Of Fire, against which Shimmering Aura offers no protection. Is there any difference between the Burning effect of, say, Fire Spray (originally Magic) compared to the Burning effect of Fiery Stab (originally Melee)? Anyway, more on topic, I agree that the reading of Stone Spikes is from the text: the effect is 6 Piercing damage from the terrain. I assume the Magic keyword is there to make the card susceptible to things like Counterspell. I also agree that this is confusing and has caught me out before.
Yes, and I feel the fact that Burning's effect is also "from the text" means we're finding a real issue here. I personally use Shimmering Aura as a "Warrior's Resistant Hide" so my Warrior can hang out with my Firestorm Wizard.* The Burning from the Firestorm definitely triggers the Shimmering Aura. * For the curious, I'm talking about a low-level saved build so I can plough through low-level adventures with Firestorm for easy loot.