If you heat up diamond away from oxygen (air completely) until the solid diamond is now gaseous, what are the gas particles made of? Just the element carbon without a charge?
Yes. Since diamond is a covalent network solid, it is made up of nothing but pure carbon atoms. Being a network solid, it is extremely stable (only at high temperatures and pressures in this case). If you have heated it enough to cause it to sublime, then the carbon atoms are in such an excited state to make it impossible for any bonds to form between them. Therefore, you would be left with gaseous monatomic carbon.