No, not all rocks are made up of one mineral. Most rocks are composed of multiple minerals. For instance, granite is an igneous rock that typically contains quartz, feldspar, and mica, while other rock types like sedimentary rocks can consist of a variety of minerals and organic materials.
The concept of "ore" refers to a type of rock that contains sufficient minerals with important elements to make it economically feasible for extraction, not merely that a rock contains multiple minerals. Therefore, a rock can still be useful even if it contains more than one mineral, and its usefulness is not solely determined by its mineral composition.
Additionally, the idea that a rock would lose its orderly internal structure by containing multiple minerals is not accurate. The internal structure of different minerals can exist together harmoniously within a rock.