A diamond polycrystal comprising more than or equal to 99% by volume of a diamond component, wherein the diamond polycrystal is composed of a plurality of diamond grains having an average grain size of less than or equal to 30 nm, wherein the average grain size of the diamond grains included in each of the diamond polycrystals is determined by an intercept method employing a SEM, in which: the diamond polycrystal is observed using a SEM to obtain a SEM image; a circle is drawn on the SEM image and then eight straight lines are drawn from the center of the circle to the outer circumference of the circle in a radial manner in such a manner that intersecting angles between the straight lines become substantially equal to one another, wherein the observation magnification and the diameter of the circle are set such that the number of diamond grains on each straight line becomes about 10 to 50, wherein the number of crystal grain boundaries of the diamond grains crossed by each of the straight lines is counted; the length of the straight line is divided by the number thereof to find an average intercept length; the average intercept length is multiplied by 1.128, and the resulting value is regarded as the average grain size; and wherein three SEM images captured in one sample at separate portions are used, an average grain size is found for each SEM image, and the average value of the three average grain sizes obtained is regarded as an average grain size; and the diamond polycrystal has a carbon dangling bond density of more than or equal to 10 ppm, wherein the carbon dangling bond density is a value indicating a ratio of the number of unpaired electrons of carbon atoms relative to the total number of bonding hands of carbon atoms forming the diamond grain, wherein the carbon dangling bond density in the diamond polycrystal is: measured using an ESR device; wherein a differential curve of an ESR spectrum corresponding to an unpaired spin of a carbon dangling bond is measured, then, the differential curve is integrated twice to obtain a signal strength; and the carbon dangling bond density is calculated from the signal strength. the diamond polycrystal is composed of a plurality of diamond grains having an average grain size of less than or equal to 30 nm, wherein the average grain size of the diamond grains included in each of the diamond polycrystals is determined by an intercept method employing a SEM, in which: the diamond polycrystal is observed using a SEM to obtain a SEM image; a circle is drawn on the SEM image and then eight straight lines are drawn from the center of the circle to the outer circumference of the circle in a radial manner in such a manner that intersecting angles between the straight lines become substantially equal to one another, wherein the observation magnification and the diameter of the circle are set such that the number of diamond grains on each straight line becomes about 10 to 50, wherein the number of crystal grain boundaries of the diamond grains crossed by each of the straight lines is counted; the length of the straight line is divided by the number thereof to find an average intercept length; the average intercept length is multiplied by 1.128, and the resulting value is regarded as the average grain size; and wherein three SEM images captured in one sample at separate portions are used, an average grain size is found for each SEM image, and the average value of the three average grain sizes obtained is regarded as an average grain size; and the diamond polycrystal is observed using a SEM to obtain a SEM image; a circle is drawn on the SEM image and then eight straight lines are drawn from the center of the circle to the outer circumference of the circle in a radial manner in such a manner that intersecting angles between the straight lines become substantially equal to one another, wherein the observation magnification and the diameter of the circle are set such that the number of diamond grains on each straight line becomes about 10 to 50, wherein the number of crystal grain boundaries of the diamond grains crossed by each of the straight lines is counted; the length of the straight line is divided by the number thereof to find an average intercept length; the average intercept length is multiplied by 1.128, and the resulting value is regarded as the average grain size; and wherein three SEM images captured in one sample at separate portions are used, an average grain size is found for each SEM image, and the average value of the three average grain sizes obtained is regarded as an average grain size; and the diamond polycrystal has a carbon dangling bond density of more than or equal to 10 ppm, wherein the carbon dangling bond density is a value indicating a ratio of the number of unpaired electrons of carbon atoms relative to the total number of bonding hands of carbon atoms forming the diamond grain, wherein the carbon dangling bond density in the diamond polycrystal is: measured using an ESR device; wherein a differential curve of an ESR spectrum corresponding to an unpaired spin of a carbon dangling bond is measured, then, the differential curve is integrated twice to obtain a signal strength; and the carbon dangling bond density is calculated from the signal strength. measured using an ESR device; wherein a differential curve of an ESR spectrum corresponding to an unpaired spin of a carbon dangling bond is measured, then, the differential curve is integrated twice to obtain a signal strength; and the carbon dangling bond density is calculated from the signal strength.