Provided is a process for producing isolated graphene sheets from a supply of coke or coal powder containing therein domains of hexagonal carbon atoms and/or hexagonal carbon atomic interlayers. The process comprises: (a) subjecting the supply of coke or coal powder to a supercritical fluid at a first temperature and a first pressure for a first period of time in a pressure vessel and then (b) rapidly depressurizing the supercritical fluid at a fluid release rate sufficient for effecting exfoliation and separation of the coke or coal powder to produce isolated graphene sheets, wherein the coke or coal powder is selected from petroleum coke, coal-derived coke, mesophase coke, synthetic coke, leonardite, anthracite, lignite coal, bituminous coal, or natural coal mineral powder, or a combination thereof.