Finding rocks to work in mass part of RMS

I'm collecting rocks I find around my property and I'm just wondering about size, type and how many rocks to put in the mass of an RMS. Some are from rock outcrops and look like the kind you can split open and find fossils in (very flat). I don't think those would be good because they would tend to have air gaps when you installed them with the cob. I think I have more than enough already and the cob would just be the 'glue' that holds them together. If I have more rocks, then couldn't I just use concrete instead of the cob? Are the bigger/more rocks going to increase the 'flywheel effect'?

Hi Dan;
As a general rule you want more rock, the bigger the better.
The cob is only for air gaps. It is the glue that holds it all together.
Your flat rock, if it has hollows inside is less desirable than a dense rock, but any rock is better than cob.

You can use concrete over the mass or as the mass, its just that...   A) it costs money and B) it is permanent.  
Rock and cob can be removed and reused.  

Oh, OK, that makes sense.

Even fossiliferous sedimentary rock is not going to have measurable hollows in it, but it may be less dense than other kinds of rock. The denser the better, but as Thomas says, any rock (except pumice ) is better than cob for mass. The cob is just to fill all gaps between rocks and cover them nicely.