I recently saw a garden show (Ken Basteda) of the Bay Area, and he built a quick and simple outdoor BBQ made of volcanic rock and outer material was flagstone. It is considered a Mexican Fire Pit, in that it isonly 3 feet high, and able to sit around the fire and roast marshmellows. I am considering it myself. The plan was to draw the ring (approx 5 feet) in diameter, and cut out the grass of the second ring. This leaves a ring of grass on the inside. The outer ring is where you would pour cement. Then build the circle of cement with volcanic rock. Then flagstone is attached to the ring top, and sides, for a finished look. Add the grill inside, and you are ready. They also laid flagstone around the pit on the ground as a finished touch. Great conversation to have around a low, but functional fireplace.