The GR1 defect can create blue color in diamonds when nitrogen concentrations are low.

• A. Only in diamonds with low nitrogen concentrations
• B. When nitrogen concentrations are high
• C. In diamonds with high nitrogen content
• D. In stones with high boron content

Answer: (A)

The GR1 defect is a single vacancy color center in the diamond lattice. It creates blue coloration when it exists in a lattice that has very little nitrogen impurities. Nitrogen atoms in the lattice form other color centers that interact with or mask the absorption caused by GR1, so as nitrogen content increases, the distinct blue color from GR1 is suppressed or altered. In diamonds with low nitrogen, GR1 stands out as the dominant defect-related absorption, yielding the blue hue. If nitrogen concentrations are high, those competing centers (such as nitrogen-related ones) dominate the color, and the blue from GR1 is not observed. That’s why the blue color from GR1 is described as occurring only in diamonds with low nitrogen concentrations.