Spoiler
Didn't we already know that Dragonstone had shit loads of dragonglass in it? Feel like that was referred to previously so not sure how much of a revelation Sam's discovery was.
Stannis: I'm told you killed a white walker
Sam: I did, your grace
Stannis: How?
Sam: A dagger made of dragonglass
Stannis: Dragonglass?
Sam: What the maesters call obsidian
Stannis: I know what it is. We have it in Dragonstone. Why would obsidian kill a walker?
Sam: I don't know [rambles on]
S05,E05
It should have been looked into sooner really!