THE CAUSE OF COLOR IN ORANGE GEM DIAMONDS

Naturally colored orange diamonds don’t often command the attention that red or blue diamonds do, but as a pure hue, their rarity surpasses that of most other diamond colors. Orange color can occur in diamond as a result of two mechanisms: single substitutional nitrogen atoms of an appropriate concentration range or a 480 nm absorption band. While significant research has been performed on individual nitrogen atoms in diamond, very little is known about the 480 nm band, despite the fact that it is most often responsible for pure orange hues in gem diamonds. It has been loosely attributed to oxygen substitution in the diamond lattice, but never directly proven. The 480 nm band is always accompanied by an unusually variable fluorescence response including yellow to orange fluorescence under long- and short-wave ultraviolet (UV) excitation and red fluorescence under strong blue or green visible excitation. Yellow phosphorescence to short-wave UV light is also commonly witnessed from diamonds with this absorption band. At low concentrations, absorption from the defect gives rise to yellow color that shifts to orange at higher concentrations. Even more remarkable is a distinctive broadening of the 480 nm band with heating that temporarily causes the diamond color to shift from yellow to orange. If the 480 nm band occurs in combination with a broad 700 nm band that correlates with hydrogen impurities, a somewhat unique type of brownish or grayish greenish diamond known in the trade as a “chameleon” is formed. The aforementioned broadening of the 480 nm component with heat gives this collectors’ stone its well-known temporary color changing property (from green to yellow or orange). While not well understood, this unusual defect in orange diamonds is definitely rare and highly valued as evidenced by the sale of a 14.82 carat vivid orange diamond (known as “The Orange”) for $2.4 million per carat in 2013.