Arthur C Clarke claimed to use a very large version of something similar to accelerate the evolution of an entire species, so there’s that.
Tiny thin one here for $450. https://www.2dsemiconductors.com/hopg-graphite/#description
graphite-moderated nuclear reactors. https://en.wikipedia.org/wiki/Nuclear_graphite
Yes, if it was 300mm in diameter, there is a lot you could do with that:) eg huge sheets of graphene for electronics.
I thought of one way it could be done. Graphite couples remarkably well to magnetron radiation. You can put locktite in a microwave to see the effect it has. If the radiation was introduced gradually or if the wavelength was controlled, in an atmosphere of methane, methane will give up carbon to graphite because it has a higher affinity for electrons than hydrogen. Its only a question of whether this method would work for growing a seed or if the growth is just too chaotic.