NATURAL COLOR IN GEM DIAMONDS: FROM THE LOWER MANTLE TO THE UPPERMOST CRUST

Gem diamonds are highly valued due to their rarity and association with prestige and royalty. While most people imagine large, perfect, colorless crystals when they think of the finest diamonds, the rarest and most valuable stones are those with strongly saturated blue, red, and green colors. Naturally colored diamonds in these hues often bring over a million dollars per carat at auctions. Given the desire for perfect, flawless diamonds, it is somewhat ironic that the rare colors that make these diamonds so valuable are caused by atomic imperfections (or defects) that are imparted to the diamonds at various stages from crystallization in the deep mantle to rapid transport to the surface to interaction with fluids in the shallow crust of the Earth. Inclusions identified in many blue diamonds indicate that they grew in the transition zone or lower mantle, where boron impurities were incorporated to create their color. Interestingly, no significant source of boron exists in the mantle, suggesting recycling from the surface through subduction. The finest red diamonds are thought to be colored by plastic deformation that occurred deep in the Earth after crystallization and possibly as the diamonds were being transported upward by fast-moving kimberlite magmas. Similar mechanisms produce red, pink, and brown color, but only a select few diamonds end up with red or saturated pink colors for reasons that are poorly understood. Finally, most of the nicest green diamonds ever discovered derive their color from radiation damage imparted by relatively low temperature radioactive fluids circulating in alluvial deposits in the shallow crust. The green color is generated when carbon atoms are knocked out of their lattice positions to create vacancies that are only stable at shallow crustal temperatures. While these are the most valuable colors of natural gem diamond, every color of the rainbow exists and is a product of combinations of natural processes that occur at varying depths within the earth. Understanding these defects helps the jewelry industry put into context the rarity and value of these beautiful gems.