Blue HPHT diamonds often phosphoresce what color?

• A. Red
• B. Yellow
• C. Green-blue
• D. Orange

Answer: (C)

Phosphorescence is the lingering glow that remains after the excitation source is removed, caused by energy being trapped in defect centers within the diamond and released slowly over time. In blue HPHT diamonds, the boron-related centers that give the blue color also create emission centers that tend to release energy in the green-blue part of the spectrum when the light is off. That combination produces a green-blue afterglow, which is why this color is commonly observed for these stones. If different impurities or defect configurations were present, the afterglow could shift toward red, yellow, or orange, but green-blue is the typical phosphorescence for blue HPHT diamonds.