In all honesty, Geralt only means to show her in return, the crystal, so the keen shine in her eyes when she sees it surprises him a little. Not that it's unwelcome. If she finds some more value to it than he intended, all the better.

"Next time, I'll hide it just for you," he replies.

He tips his head. This is where his knowledge ends: when it comes to specifics of a spell. "I couldn't tell you more. I'm not a mage. But in here—" He gestures around them. "—I imagine you'll find your own method."

She may not have a megascope or the requisite spell, but she won't need one, in the same way he knows he won't actually need a...TV...to look at the contents of this tape she's given him. Such is the nature of a world with no limits.