Rocks as filler inside a cob wall

Hi folks,

This is my first time working with cob (as the base for a rocket-core-powered mud oven). I have to buy sand, but I have loads of local basalt rocks (in lots of different sizes). What I'm wondering is this. Can I use some of those rocks as filler in the middle of my cob wall? This will mean less sand for me to buy, and less work because I won't need to mix as much cob.

If this is an OK approach, how much rock can I get away with before compromising the structural integrity of the cob? My gut says if I keep it less than about 50% of the thickness of the wall, and make sure that the cob on one side of the rocks has plenty of places where it connects to the cob on the other, I should be OK.

What do you guys think?

Something like this: