Any glue made for porous materials should work but I'd use clear Gorilla Glue. Wet both surfaces and use a very small amount as it expands. If it does expand out of the mend, you could sand the line down with fine sandpaper.
I had a wild boar skull from a hog I shot in the head, when it was boiled off 3 three pieces of the skull came off. After it was clean I glued them back with super glueone piece at a time when that one dried the next till I had it out together. That was 6 years ago and it is still together.