Weight
The base color weight. When rendering diffuse surfaces, it is very important that the normals of the geometry face in the right direction. In the example below, you can see the difference between normals that are facing inwards in the wrong direction (left side) versus those that are facing correctly in the outwards direction (right side).
Surface Normal Direction
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Color
The base_color sets how bright the surface is when lit directly with a white light source (intensity at 100%). It defines which percentage for each component of the RGB spectrum does not get absorbed when light scatters beneath the surface. Metal normally has a black or very dark base color, however, rusty metal's need some base color. A base_color map is usually required.
Diffuse file texture -> base_color
Diffuse Roughness
The base component follows an Oren-Nayar reflection model with surface roughness. A value of 0.0 is comparable to a Lambert reflection. Higher values will result in a rougher surface look more suitable for materials like concrete, plaster or sand.
Metalness
With metalness 1.0 the surface behaves like a metal, using fully specular reflection and complex fresnel. For a perfectly sharp and mirror-like reflection, increase metalness to 1, and reduce specular_roughness to 0. The base_weight should also be set to 1. When metalness is enabled, the specular_weight and specular_color only control the edge tint, while the base is still affected by roughness. The metal appearance is controlled using the base_color (facing) and specular_color (edge tint) parameters. These are automatically translated to physical η and κ values, to achieve the same look but with easily tweakable and texturable colors. Some examples of real-world values for metals can be found below. Metalness values between 0.0 and 1.0 can be used to texture surfaces like rusted iron, where different areas of the surface can have more reflective clean metal and more diffuse rust. PBR metalness maps from applications like Substance Painter can be connected to this parameter.
Rusted iron texture -> metalness
Shaders using metalness textures
Base Color Specular Color Aluminium (Al) 
0.912 0.914 0.920 
0.970 0.979 0.988 Copper (Cu) 
0.926 0.721 0.504 
0.996 0.957 0.823 Gold (Au) 
0.944 0.776 0.373 
0.998 0.981 0.751 Iron (Fe) 
0.531 0.512 0.496 
0.571 0.540 0.586 Lead (Pb) 
0.632 0.626 0.641 
0.803 0.808 0.862 Mercury (Hg) 
0.781 0.779 0.779 
0.879 0.910 0.941 Nickel (Ni) 
0.649 0.610 0.541 
0.797 0.801 0.789 Platinum (Pt) 
0.679 0.642 0.588 
0.785 0.789 0.784 Silver (Ag) 
0.962 0.949 0.922 
0.999 0.998 0.998
Metal presets
Some metal presets are available in the UI with the ability to save/load custom presets. These presets in effect adjust the Base Color and Specular Color as per the Metalness table above.