What does the HPHT treatment do to a diamond, and what post-treatment step is commonly required?

• A. It rearranges defects in the crystal lattice; repolishing is always required after HPHT.
• B. It adds new impurities to the lattice.
• C. It changes only the surface texture with no lattice changes.
• D. It coats the diamond surface with a protective layer.

Answer: (A)

HPHT treatment alters a diamond by rearranging defects in the crystal lattice, especially nitrogen-related centers, which can change the stone’s color. Because this process can affect the near-surface layer and leave subtle surface changes, a repolish is commonly required after HPHT to restore the facet finish and maximize brilliance. The other ideas don’t fit: HPHT does not introduce new impurities, it’s not limited to changing only surface texture without lattice changes, and it doesn’t coat the surface with a protective layer.