He sees her face fall, immediately concluding that it must be about the oath. She hadn't even talked to him for, like, two weeks after she found out; of course that has to be it. He's about to say something about it, but Natalie gets to it first, and he just rolls his eyes good-naturedly.

"So, funny thing. We were in the middle of a fight - you know, being totally heroic and all - and the person comes out of nowhere and nearly kills me. The weird thing, though, is that right when he nearly got me, this... shield thing came out of nowhere, stopped him dead in his tracks. Last I checked, I didn't have any force fields in my belt, but then this burned out rock appeared in my pocket," he says conversationally, pulling it out. "Must have been in with some nuts and bolts I had lying around. Maybe you could tell me what this rune means, since you're good at that stuff?"

Translation: you could have just told him, Natalie, dios mio.