RMH not using cob, but other materials

I was just watching one of the videos about building a RMH with a wooden box and pea gravel and stone for the "cob".  
Can lava stone be used for the "mass" as well?
Suggestions for other materials that can be used to build in a single wide mobile home?

Thanks!

Welcome Moira,
If the lava stone has little voids like an Aero chocolate bar then they would be more insulative then heat gathering which is opposite of what you are looking for when it comes to a heat storage mass.
The cob can be used alone as a mass material but when it is only used as a mortar to fill the air gaps between more dense items like rock or urbanite (concrete chunks), the heat storage capacity can be increased. So really, you could say that you've built your mass with rock (or whatever) rather than cob.
Here is a previous discussion of some of the materials used in a RMH : What is the best material for a heat bank?

Thank you so much Gerry! :}  Still working on getting the bricks for building the core and still need to find out our building codes here in ME.  
Ciao for now!