Stonework means the stone parts of a building or structure — the walls, arches, window frames, steps, columns and carved decoration, all taken together. When a guide says 'look at the stonework above the door', they mean the worked and fitted stone itself, not one single block. The word carries a sense of craft: stonework is stone that someone has cut, shaped and laid, which is why you meet it constantly in talk about old churches, castles, bridges and city buildings, and in the language of repair — 'the stonework is crumbling', 'they cleaned the stonework'. It is uncountable, so no 'a stonework' and no plural. It can also mean the work of building with stone as an activity, though 'masonry' is the more usual word for that. Neutral in tone and used by ordinary people and specialists alike; it just happens to come up most where old buildings do.