"I am so sorry!"
The tears ran in rivulets down the purple unicorn's face as she trotted a circle, unable to look at her bed-bound companion. If Rarity could garner the ability to sit up, she imagined she'd see a trench in the tile. If Twilight kept going for another hour or two, might she go all the way through and get to the hospital's first floor?
"I studied the spell just like you told me. I thought I had it all figured out! And, to be fair, I did, but I vastly overestimated the amount of charge it needed. I had no idea it would be so efficient. I know you told me it was easy and that I shouldn't worry about power but I got your hints all wrong and I'm so stupid, oh, how could I let this happen?"
Really, it was astounding how long Twilight could go without taking a breath. Rarity imagined only Pinkie could beat it.
"Twilight."
"I promise, I won't use the spell again. I still can't believe it decided you were a legitimate target! I mean, how? Why? It doesn't make any sense!"
"Twilight."
"I tried to turn it off, but it wouldn't stop, just like you described. Is it something I did wrong? It doesn't do that to you every time, does it? This thing might qualify as a tool of war in the wrong hooves, and I just cast it willy-nilly like a colt who found his father's spear!"
"Twilight."
"And then Spike had to see it, and there was so much blood, and you were screaming. Oh, Celestia, what if I've traumatized him for life? He's a dragon, he's got a lot of life to be traumatized in. He'll be in therapy for decades!"
"Twilight Sparkle!"
The librarian, royal protégé, and magical genius whirled around to face her, stumbling back as if hearing her name were a hammer blow. She blinked wide eyes at the bed-bound Rarity, who was halfway sitting despite her weariness and the pain such a position brought her. "W-what? No, Rarity, you shouldn't try to get up!"
The dressmaker fell back on the bed with a relieved sigh, taking a moment to gently rub at the thick layer of bandages on her sore flank. "Yes, darling, I know, but I had to get your attention. Now, if you'd please stop beating yourself up over this, we can get to the part where I forgive you for an honest mistake."
"B-but, but, I hurt you. I might have even scarred your cutie mark." Twilight wiped her eyes and took a step closer. Her hoof went up to touch her own bandaged horn. "I shouldn't have tried to use your spell."
"Oh, pish-posh. I'm flattered you thought it worth the effort, and I am the one who taught it to you. Besides, now I can say with absolute confidence..." Rarity flicked her mane and shot Twilight the smuggest of grins.
"I really am a gem among ponies."
I just read it, and... I'll think about it and reread it again later and see if I put two and two together then. Or if any other readers do before me.
11836626
I'll given it some time and if nobody catches it I'll probably append to the Author's Note the explanation. Dunno how long I'll wait for that though.
Ah. A cutie mark is, on a metaphysical level, the thing in itself. At least, it is for the purposes of some magic. This is why you don’t overcharge spells casually, folks.
So, Rarity taught her the gem hunting spell and since it drags you to the gems it dragged Twilight to Rarity's cutie mark. Right??? Horn first?
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As we see in the show, it doesn't do that if you know what you're doing. But Twilight, y'know, never tried it before, so she ended up putting way more power into it than it needed. So... yeah. Dragged horn-first to the nearest gem.