I noticed that he is just using a saw blade in the reciprocating saw, that is what led me to think that it is like CSR Hebel Artists carving Block.(Aeated Cement)
Its just the colour that has me stumped and also the outside texture and shape as though it was made in a wheelbarrow or cast in a mould, you dont see a flat or smooth area from the start, but it is my guess that he made it.
There is a reply from someone saying cement, but I think that is a guess.
fine sand and cement mix that he pumps compressed air into,through like a flow table to aerate the mix.
something like that could end up like a hebel. and he has put oxide into the mix to give it that colour.
I have heard of that but never seen someone do it or what it would turn out like
I'm a bonsai slightly better than a newb