@neko259 Correct however, Erlehmann has made an obsidian mod which adds obsidian and cobble from lava & water which can be found here:
http://c55.me/minetest/wiki/doku.php?id=modsEDIT: one thing to note to get the obsidian mod to work you will need to change any default mod references to include "default:" i.e. instead of just "water_flowing" it should be "default:water_flowing" same goes for any other default mod related content in there like lava_source, lava_flowing, water_source etc.