Abstract: Rrecent advances in diamond synthesis are reviewed. Artificial diamonds synthesized under high pressure and high temperature has led to wide usage of such diamonds in industry. Low temperature coating of diamonds has been studied for some time and chemical vapor deposition has been developed. In our group dense carbon dioxide is reduced by alkali metals (Li, Na, K) to produce colourless large diamond crystals at 800 atm and 400 ℃. Diamonds readily burn in air, forming carbon dioxide. This is the first time that the reverse conversion of carbon dioxide to diamond has been realized. Carbonates are important candidates as potential hosts for carbon in the Earth's mantle and crust, It is known that the mantle in the past has been strongly reducing, based on the thermodynamic analyses of diamond inclusions. It is suggested that diamond within the Earth could be produced from carbon dioxide wherever carbon dioxide exists and conditions (eg. temperature, pressure, and reducing environment) are satisfied.