What color is diamond dust (the crushed gem, not the weather phenomenon)? My friend claims it is black but I believe it to be white (or clear, but appears white due to light interaction).
Diamonds can come in many different colors; the white ones which are usually associated with the word “diamond” are just one (admittedly the most popular) variant of the gem. So I suppose diamond dust can come in different colors as well.
It’s true that the white color of many things, such as snow or styrofoam, is the result of the light being refracted by the many tiny bubbles the material is composed of. But I don’t think diamond dust is fine enough to produce this phenomenon.
OTOH, I’m just a layman giving a WAG here, so I might very well be wrong. I’m just presenting some thoughts for further discussion.
Diamonds come in a variety of colours, as do the pulverised versions. Dust produced by crushing up gems like e.g. the Cullinan diamond would be clear and sparkly. However the majority of diamonds found are not gems and are typically small crystals of blackish appearance, used as abrasives - this is probably what your friend is describing.
According to wiki industrial diamonds are known as bort, but I think I have also seen that term used generically for diamond grit, including the small material left from cutting/polishing.
Black diamond is found in many places and is mostly used for industrial cutting. Your friend is talking about the black diamonds.