The Kyber Mirrors were an attraction under the Dome of Deliverance on Jedha. They were opened by the Priests of Phirma only on Reflection Day. It was said that the mirrors could show you a glimpse of your future path, to better understand what the Force might have in store.
There were three mirrors, which were sheets made of polished kyber that sprouted from the ground like huge spearheads. The mirrors were covered in a dull, speckled platina and shone with an inner glow. All mirrors had a small wooden platform before them for the pilgrims to stand on.
While it has yet to be officially confirmed, some recent Star Wars stories may have told us more about the strange mirror Rey visits on Ahch-To in The Last Jedi. The Visual Dictionary for the film calls that room the Mirror Cave, and I think it’s likely the mirror itself is made up of kyber crystal.
It makes sense that one of the earliest Jedi Temples would have so much kyber on it. And if it was truly the first Jedi Temple, they’d have to get their lightsaber crystals from somewhere. The High Republic era recently mentioned that Jedha, another early site for the Jedi, also had kyber mirrors, and then the Life Day Treasury expanded upon that idea, saying that people would come from across the galaxy to look into the mirrors for a glimpse into their future. The Star Wars Visions tie in novel, Ronin, also brought up the concept of kyber mirrors, and they served basically the same function.