Characters of Brown Spot on Diamond from Hunan Province and Its Implication

There are many brown spots on the diamonds from Hunan Province, which is one of the three important diamond places in China. The brown spots are yellow, yellowish brown, brown and black on the etched surfaces of any type of the diamond. CL colour of the brown spots is dark, but even yellowish green in the surroundings. Raman spectra show that the centers of the spots associate with vacancy, the characteristic peak value of the diamond increases and the full width at half maximum decreases from the center to rim of the spot. The colour of spot is caused by radiation of natural radioactive material. The alluvium bearing diamonds were experienced thermal metamorphism, and reformed by modern river again.