Scottish Galaxy Stones are really just slow cooled lava with inclusions in them. The lava cools while the bubbles of oxygen beneath it move upwards to escape. Everything solidifies and you get this holey rock material. 
Over time, sediment gets trapped in the holes. In this case calcites and silica minerals like chalcedony. They form little crystal bubbles which look like planets - which is why they're called galaxy stones.
This one is from South Ayrshire.