It's hard to tell from images available online, but it may have been anthracite or bituminous coal, which are both surprisingly hard - more like a brittle rock than, say, the charcoal you'd use in a BBQ.
True, this article claims it was varnished but I don't see any evidence for the claim. Regardless, it was a silly stunt but did show his allegiance clearly.
https://www.theaustralian.com.au/weekend-australian-magazine...