What material is my residence-adjoining common wall made from?

My house is a twin built in 1920. Most of the walls are plaster, some are drywall. Knocking on these with my knuckle yields a hollow sound, like tapping a door or window. However the center wall that divides our house is extremely hard, and when I tap on it, there is no sound whatsoever; except for some guitar sounds (mostly low strings) I can’t hear a single thing going on next door.

What is this material? It runs down the middle of our entire house. To my eyes it’s just a painted white wall!