Your appeal to the stone notwithstanding...
When talking about elephants and their trunks, let's say I want to put an elephants toy in the trunk of the car. Or I could let the elephant pick up the toy with his trunk, and have him put it in the trunk of the car.
The Bible does the same thing as in the above sentences, just with "firmament" and "firmament of the heavens. Why do you think the first FIVE times Moses used just the word "firmament" (Hebrew "raqia," which means something pounded out, like a smith pounds out metal to make it thinner and to spread it out), and then the next FOUR times he adds "of the heavens"?
It's because they're two different things, just like when talking about elephants and their trunks, putting elephant toys in the trunk of the car. Two different things are being spoken about, the firmament, and the firmament of the heavens.
One is the crust of the earth, the second is the sky.