Is this seiryu stone? Some people have said it’s obsidian

Hi I have a previous post about this but it’s buried and I’ll flag it once we can for sure identify it. Only reason why I think it’s seiryu is because it looks exactly like it when it’s dry. Circles on last picture are for small rust like spots appearing on the rock after being submerged in water for 3 days (I’m planning on using it for an aquarium).