An aqueous suspension liquid of finely divided diamond particles comprising 0.05 to 160 parts by weight of a finely divided diamond particles in 1000 parts of water, wherein; (i) the finely divided diamond particles have an element composition consisting mainly of 72 to 89.5% by weight of carbon, 0.8 to 1.5% of hydrogen, 1.5 to 2.5% of nitrogen, and 10.5 to 25.0% of oxygen; (ii) and, almost all of said diamond particles are in the range of 2 nm to 50 nm in diameters thereof (80% or more by number average, 70% or more by weight average), (iii) and, said finely divided diamond particles exhibit a strongest peak of the intensity of the Bragg angle at 43.9.degree. (2.theta..+-.2.degree.), strong and characteristic peaks at 73.5.degree. (2.theta..+-.2.degree.)and 95.degree. (2.theta..+-.2.degree.), a warped halo at 17.degree. (2.theta..+-.2.degree.), and no peak at 26.5.degree., by X-ray diffraction (XRD) spectrum analysis using Cu-K.alpha. radiation when dried, (iv) and, specific surface area of said diamond particles when dry state powder is not smaller than 1.50.times.10.sup.5 m.sup.2/kg, and substantially all the surface carbon atoms of said particles are bonded with hetero atoms, and the total absorption space of said powder is 0.5m.sup.3/kg or more, when dried. The diamond particles are very active and dispersible in aqueous liquid in stable, and have essentially same mechanical properties as that of usual diamonds.