Render Controls - Materials
Filter field can be used to filter materials list to find the exact block you wish to edit. The following section explains the available material properties.
This controls the amount of light that a block emits. For full blocks, you can calculate the Chunky emittance from Minecraft brightness by dividing by 15. For example, glowstone uses 1.0 while magma uses 0.6 by default.
Due to the way Chunky renders scenes, the brightness depends on the surface area of the block. A torch requires a much higher emittance value (and thus results in a lot of noise) to achieve the same amount of brightness as a full block. Chunky uses a default emittance of 50 for torches.
This controls the specular reflection (0.0 - 1.0). This is a mirror-like reflection of light and is perfect for glossy non-metallic materials (e.g. diamonds). A value of 0 will result in a non-reflective block, a value of 1 will make it a perfect mirror.
Some people use a small specular value on e.g. the grass block to achieve a wet surface look. While physically correct rendering of wet surfaces is way more complex, this is a decent way to render rainy scenes in Chunky.
This controls a material smoothness/roughness (0.0 - 1.0). This is an approximation of surface irregularities and leads to fuzzy or diffuse reflection. Lower values increase roughness.
Smoothness vs. roughness
Internally, Chunky uses linear roughness for its calculations. Working with roughness is pretty hard for a human, which is why LabPBR introduced perceptual smoothness. This makes it so that 0.5 feels like twice as smooth as 0.25 for a human.
You can find out more about this, including the formula to convert between perceptional smoothness and linear roughness in the shaderLABS Wiki.
The Index of Refraction controls how much slower the light travels inside the block, relative to a vacuum. This is the effect that makes water and glass "bend" the light while it enters and exits (i.e. transitions between materials).
In the real world, metals reflect in a different way than dielectric (i.e. non-metals), which is what makes them shiny. A value of 0 means that it will only have specular reflection and a value of 1 means that it will only have metallic reflection.
Metallic reflection compared to specular reflection.
Metalness vs. real world
While there is no such thing as 50% metalness for a real material, you can use this to approximate dirty metallic surfaces (and use it for other artistic purposes, of course). For example, Chunky uses metalness values smaller than 1 for oxidized copper blocks by default.