The way to get obsidian in asteria is to mix water and lava. Lava is found deep below the surface, and water is usually only found close to the surface (you need the pure, blue water.) So you'd think you'd have to dig that water all the way to the lava, right?

Well, nope.

Make a Bose Einstein Condenser. Any one will do. This gun shoots ice and freezes things.

Just find some lava, and shoot your Bose Einstein Condenser at it. The ice will melt (into water, who knew!) , mix with the lava, and make obsidian.