Abstract: This study focuses on geological, microstructural, oxygen isotopic, and thermo gravimetric investigations of Oltu stone, which is the most important turbostratic carbonaceous material in Turkey.The results of our investigations indicate that the carbonaceous Oltu stone material (specific gravity of 1.317)is not an organic material, such as jet, derived from fossilized wood. Rather, it is composed of a carbonaceous phase intermediate between amorphous carbon and crystallized graphite (termed turbostratic carbon), that is intercalated with flysch and formed by the reduction of seeping magmatic carbon dioxide during the diagenesis of Upper JurassicLower Cretaceous marine sediments. Oxygen isotope analyses (SMOW) (using EA-IRMS) of both Oltu stone (δ18O= +37.2‰to +40.8‰) and the enclosing flysches (δ18O = +10.3‰ to +12.3‰) suggest that the nodules formed during diagenesis at a temperature of around 50 ºC. However, they are enclosed in flysches whose grains are derived from rocks that formed at significantly higher temperatures, perhaps above 100 ºC.The main industrial use of Oltu stone is as host material for diamond coating, as the sp3 bonds in the material can provide nucleation sites for diamond crystals and improve the nucleation rate at the early stage of diamond deposition on turbostratic carbon.