What is the most important tool in a Machine Shop? - that would be the surface-plate.
The surface-plate is a indespensible tool in a machine shop. It establishes a reference plane from which precision measurements can be made.
The surface plate that I have in my shop is a cast-iron plate made from seasoned gray cast iron. It was rough and finished machined on a planner, and finally hand scraped to the required flatness.
Today, if you were to visit a machine shop, you would find a granite surface plate being used. Black granite is the material used today in the manufacture of surface plates. Granite surface plates can be made with a flatness of a milionths of an inch. My cast-iron surface plate has a flatness of 1/thousandth of an inch. So keep in mind how important a surface plate is, as precise machine work starts with good layout work.
Oh, just so you know, my surface plate has a fitted cover that is kept in place to protect the surface when the surface plate is not in use.