A mirror is slightly less destructive than intersection with a crystal but can also generally bust a light in fewer than ten tries.

Huh. What about glass things with a wide variety of opacities. 

Things that are only just transparent enough to not completely cover the light kill the light quickly, and less quickly the less opaque they are from there; transparent things that don't reflect, refract, or recolor the light are not very destructive.

"You realize that if you explain things to me when I ask you get the debt for them and if you just let me play around discovering them you don't?"

"I don't especially want to acquire debt."

"As long as you know." What about a cup full of water.

Disturbs the light as little but not as much as a mirror.

He takes notes on all of this.

Promise will keep replacing the lights for him for a while. She doesn't have a lot of sources of entertainment around here.

"Is there a way to make them last longer?"

"Not that I'm not already doing, but I can make them more fragile."

"How many can you sustain at a time?"

"There's no particular limit besides that if I just kept doing it for ages while you were breaking all the old ones we'd eventually converge on some number of lights existing at once."

"This would make travel at night a lot easier."

"They don't move, so only if you have me along."

"Huh. Can you attach them to an object? If you make them while you're on a moving cart do they stay in place and get left behind? Does it matter that the world moves?"

"I can't attach these to an object, they get left behind, and apparently not but I didn't learn to make them under conditions where that was a factor."

"Can you vary their size?"

"No, but I can make bundles of them." She makes a bunch close enough together that it'd be easy to miss they were several.

He will have more questions for a while but eventually he'll run out. Or have gotten into more debt than he feels comfortable with.

Apparently it's not the latter because shortly after running out of questions about the magic he says "why don't you want to explain more?"

"Well, I like you personally but your species seems kind of generally horrible probably because of the wacky constraints overshadowing every interaction you ever have and it doesn't lend itself to a high trust relationship."

"Generally horrible compared to people where you're from?"

"...I think we're higher variance. If I liked someone personally back home, and they didn't do any of a few specifically threatening things, it'd generally be safe to interact with them."

"It's safe to interact with almost everyone here as long as they have some reason to think you're - basically goodwilled yourself."

"I'm not really confident that's the case in the way I would need."