Wall materials in old house
My husband and I just bought a delightfully quirky 1930 Craftsman house in central Iowa. As we started painting and decorating, we discovered that it has the hardest walls we've ever seen. When you tap them, they don't have the hollow sound of lathe and plaster like we expected. Instead, they sound solid. It takes serious pounding to get a nail in, and the drill has some trouble with screws. On the upside, the soundproofing is amazing!
In the public areas of the house, the plaster is very thick and you can't see the joints. In the bedrooms, however, the vertical joints are clearly visible and there are lots of long stress cracks. The attached picture is from the inside of the built-in buffet in the dining room. Here, the wall material doesn't seem to have been covered. It feels hard, brittle, and rough.
Does anyone know what this is? I've looked up tons of pictures online, but can't seem to get a positive identification. I have real concerns about this being some sort of asbestos-laden material since the house is so old. I'd really appreciate any help!