No, a lump of coal is opaque and non-white.
Not all crystalline materials are opaque, for instance diamond and many others. Some crystalline materials are opaque, it's because they absorb white light, which causes a gap (from IR up to UV) in the transmission spectra.
The surface will appear white.
In my limited experience no. It's white to brown, dull in color, somewhat opaque.
Gluesticks are an opaque white.
Then, your diamond looks opaque white under a black light.
No, any color may be opaque, even white. Opaque is simply the quality of not being transparent or translucent.
An opaque white
Watercolor that has been made opaque by adding inert white pigment to it is called "gouache".
This applies not only to opaque objects. The basic idea is that white light is a mixture of different colors, and objects tend to reflect the different colors - the components of white light - in different proportions. For example, an object that reflects most of the red light but not much of the other colors will look red.
White light is all the colors of light. Which are magenta,yellow,and cyan being the main three colors that are reflected by an opaque white object
Selecting Opaque specifies that the existing picture will be covered by the white background of a selected area in Paint.
I think it would be opaque.