What is the hard scabby material that grows over wounds?

Fluid and cells: There is a mix of lymphatic fluid and cells. There are a variety of cells within the fluid. Once the fluid dries it turns into the scab you see.

Dried blood : A scab is a collection of dried blood cells and serum that sits on top of the skin surface. Its purpose is to protect the wound from contaminating materials but otherwise isn't a whole lot of help. Sometimes confused with scab is eschar. It is a leathery collection of dead tissue within the surface of the wound. It is often hard, smooth and flush with the skin surface and usually needs removed.