I had a lot of black, goopy paint full of gum arabic lying around so I thought I would try making an obsidian block. It came out much as you might expect …
It is very shiny - especially after varnishing - but the underlying red-blue-purple colours are not at all obvious in these picture.
If I was to make this properly, I think I would have patches of colour and lines of black between them, to give a faceted look. Could also have a thinner layer of black on top? Here is another look anyway
Again, shiny but nothing much more comes through here.