Here's why:

* Composition: Limestone is primarily composed of calcium carbonate (CaCO3), which is a compound of calcium, carbon, and oxygen. It doesn't contain any metals in its chemical structure.

* Properties: Nonmetallic minerals generally lack the characteristic properties of metals, such as luster, conductivity, and malleability. Limestone is typically dull, non-conductive, and brittle.

While limestone can sometimes contain traces of metallic minerals, its primary composition makes it a nonmetallic mineral.