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.
Since it appears green in white light, it reflects the green part of the spectrum, absorbing all the other colors. In red light, it would not reflect any color, thus appearing black. Opaque objects absorb some of the light, and reflect the rest.
Emulsions typically appear white or opaque when light shines through them. This is due to the scattering of light by the dispersed particles in the emulsion, which causes the light to be reflected in multiple directions.
When white light shines on an opaque violet object, the object absorbs most of the colors in the white light spectrum except violet. The violet light is reflected off the object, making it appear violet to our eyes.
An opaque white
White is not transparent; it is an opaque color. Transparency refers to the ability of light to pass through a material. In the case of white, it reflects all visible wavelengths of light, making it appear white.
Egg whites are typically clear or translucent, but can appear white when cooked or whipped. Sodium hydroxide is a white solid.