The "metalness" value, determines the degree of metallic lustre of a material represented in renders.
Metalness Value: Metalness value, in essence, determines how much metallic lustre an otherwise solid colored material can produce in the renders.
Customizing Metalness Value: The ability to customize the metalness value is a valuable tool in achieving the desired appearance of materials in your renders. When working with materials that exhibit a high degree of reflection, such as polished metal surfaces, you may opt for a higher specular value, often ranging from 0.5 and above. This enhances the reflectivity and shine of the material, mimicking real-world polished metal surfaces.
In contrast, when dealing with matte surfaces, where light reflection is minimal, you can decrease the metalness value. Below is a comparison chart of the effect of metalness on a surface at various degrees:
Effect of Roughness: When creating relatively rougher/matte metal surfaces (such as rusty, corroded or textured metallic surfaces), a roughness value greater than zero can be simultaneously assigned. Below is a comparison chart that best explains the combined effects of metalness and roughness values on a surface: