PURPOSE: To objectively measure a color of a brilliant-cut diamond, and to appreciate a color grade, by measuring a spectral transmission spectrum of a diamond, and obtaining 3 stimulus values X, Y and Z of a color from its spectral transmission spectrum.

CONSTITUTION: In a diamond color measuring device 1, a light source part 2 consisting of a halogen light source lamp 3 and a prism spectroscope 4 irradiates a monochromatic light beam 5 to the whole table surface side of a brilliant- cut diamond D. The diamond D is held by a holder 6 provided with a conical recessed part 7 conforming with a pavilion P. The holder 6 is fix-provided on a moving base 8 together with a standard white plate 9. A photodetecting part 10 consists of an integral ball 12 and a photodetector 13, and integrates and detects a light beam 10 emitted through the inside of the diamond D. A measuring part 15 controls the spectroscople 4 and obtains a spectral transmission spectrum of the diamond D from a data of a photodetecting part 11. An operating part 16 obtains 3 stimulus values X, Y and Z of a color from a data of the measuring part 15, knows a color grade, and also enters a coordinate point in a form of a plotter 17.