Do you even know what Diamonds are?

1) If diamonds dissolved when introduced to a C02 atmosphere then there would be no practical way to handle nor create them.

2) 'Fake' or cultured gem-Diamonds of the 21st century are manufactured by either CVD or HPHT (Chemical Vapor Deposition or, High Pressure, High Temperature respectively) CVD is used to produced 'Gem' Quality Diamonds en masse at an increased albeit profitable expense. HPHT is used to create diamonds for industrial use, this is a rather old method it operates very efficiently.

3) You can make 'diamond' micro dust with borax and Pipe cleaners. Arguing nomenclature is fruitless but the point being, diamonds are assigned arbitrary value and significance.

4) His method is crude, but it follows procedure. Graphite is used to cause a reaction between a Hydrocarbon (like methane) and atomic hydrogen once the latter is turned into a gas which then should bond with the Hydrocarbon and then once it dissipates you have a Diamond. For a gem quality diamond to be produced you need to have a temperature range of 900 - 1200 degrees celsius.