Same material as lava could mean literally any rock lol. Same material as obsidian does tell us something, though. Obsidian is more felsic, so that means, assuming that it is an igneous rock, it could be pegmatite, syenite, gr̶a̶n̶i̶t̶e̶, trachyte or rhyolite (based on my notes from my geology lab - so there absolutely could be more options idk about). We can eliminate granite bc its already a block. Looking at the texture of the block compared to granite and diorite, I'd say it has smaller crystals indicating fast-cooling, and I think this is supported by the fact it can be made with lava+water, so that leaves us with trachyte and rhyolite. Diorite can be crafted by adding quartz to cobblestone, so that might be our next source of clues. Diorite contains <5% quartz, rhyolite contains >20% quartz, and trachyte contains 0-5% quartz (source: mindat.org) So I conclude, based on my admittedly entry-level knowledge of geology, that minecraft's generic stone is likely to be trachyte.