What's this black render?

Hey folks,
So I'm currently in the process of stripping the walls on one of the bedroom's of my c.1903 South Wales terrace house in order to insulated them better. I'm hoping to take them back to stone and have managed to take the plasterboard off one of the walls so far, but I was wondering what this black stuff covering the stones is? It sounds quite hollow to tap, and crumbles with a bit of a scrape.

Thanks!

https://imgur.com/hkerPg7
https://imgur.com/rmQ1ria

 

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