They probably see it. Let's make up a story without any proof that it's real:
In some place in the countryside, they see a light moving in the sky and a not very far explosion. (The ones that see a nearby explosion are dead.) Imagine something like the meteorite at Russia a few years ago. (From the videos it looks like a nice show if it doesn't land on your head.)
The local governor/priest/whatever heard about it and send a crew to the site, let's say 5-10 persons. They found some molten glass and a strange metal stone, that is like bronze but has a weird color.
They share the story for generations, if you are lucky someone they wrote an inform for the pharaoh. Everyone think they are nuts, but a they have this weird stone.
Wait a few centuries, and repeat this a few times, and now you have a collection of weird stones and/or the weapons/tools made with them, and they share a similar story. So the story is not long so weird.
Anyway, the knife made with the stone is better than the usual knives, and it has a nice story, so someone send it to the pharaoh.