We report new δ 13 C data and N content and aggregation state values for microdiamonds recovered from peridotites and chromitites of the Luobusa ophiolite (Tibet) and chromitites of the Ray-Iz ophiolite in the Polar Urals (Russia). All analyzed microdiamonds contain significant nitrogen contents (from 108 to 589 atomic ppm ± 20%) with a consistently low aggregation state and show identical infrared spectra dominated by strong absorption between 1130 cm –1 and 1344 cm –1 , and therefore characterize type Ib diamond. Microdiamonds from the Luobusa peridotites have δ 13 C (PDB) values ranging from −28.7‰ to −16.9‰, and N contents from 151 to 589 atomic ppm. The δ 13 C and N content values for diamonds from the Luobusa chromitites are −29‰ to −15.5‰ and 152–428 atomic ppm, respectively. Microdiamonds from the Ray-Iz chromitites show δ 13 C values varying from −27.6‰ to −21.6‰ and N contents from 108 to 499 atomic ppm. The carbon isotopes values have features similar to previously analyzed metamorphic diamonds from other worldwide localities, but the samples are characterized by lower N contents. In every respect, they are different from diamonds occurring in kimberlites and impact craters. Our samples also differ from the few synthetic diamonds we analyzed, in that they show enhanced δ 13 C variability and less advanced aggregation state than synthetic diamonds. Our newly obtained N aggregation state and N content data are consistent with diamond formation over a narrow and rather cold temperature range (i.e., <950 °C), and in a short residence time (i.e., within several million years) at high temperatures in the deep mantle.