Formation and growth thumb|Age zones in a diamond Diamonds in the mantle form through a metasomatic process where a C–O–H–N–S fluid or melt dissolves minerals in a rock and replaces them with new minerals. (The vague term C–O–H–N–S is commonly used because the exact composition is not known.) Diamonds form from this fluid either by reduction of oxidized carbon (e.g., CO2 or CO3) or oxidation of a reduced phase such as methane.
Formation and growth thumb|Age zones in a diamond Diamonds in the mantle form through a metasomatic process where a C–O–H–N–S fluid or melt dissolves minerals in a rock and replaces them with new minerals. (The vague term C–O–H–N–S is commonly used because the exact composition is not known.) Diamonds form from this fluid either by reduction of oxidized carbon (eg., CO2 or CO3) or oxidation of a re,duced phase such as m.ethane.