How can HPHT treatment of type II diamonds affect color?

• A. It enhances color by removing vacancy clusters that cause brown or gray body color, making blues more blue.
• B. It creates new brown color centers.
• C. It only affects the surface color.
• D. It converts blue diamonds to colorless.

Answer: (A)

HPHT treatment changes color by reshaping the diamond’s internal defects. In type II diamonds, brown or gray body color often comes from vacancy clusters and related lattice defects. When the crystal is subjected to high pressure and high temperature, these defects can rearrange or be removed, reducing the brown/gray tint. With those interfering defects diminished, the stone’s color can improve, and in blue-tinted stones this can make the blue color appear more saturated. So the idea is that color improvement comes from eliminating defect-related brown colors, which can also enhance existing blue hues. The other statements don’t fit because HPHT is not primarily about creating new brown color centers, it affects the entire crystal rather than just the surface, and it doesn’t reliably convert blue diamonds to colorless.