Which statement best describes Type IIa diamonds?

Which statement best describes Type IIa diamonds?

• A. They have no easily measurable nitrogen or boron impurities.
• B. They contain boron atoms replacing carbon in the lattice, with negligible nitrogen impurities.
• C. They have high nitrogen content.
• D. They have boron impurities but not nitrogen.

Answer: (A)

Type IIa diamonds are defined by an almost complete absence of impurities, especially nitrogen and boron. Because nitrogen causes certain absorption features that create color in many diamonds, its near absence makes Type IIa stones unusually colorless and often highly valued for their transparency. Boron is what gives Type IIb diamonds their blue hue and, in some cases, electrical conductivity; since Type IIa lacks boron as well, it remains colorless or nearly so. So the best description is that they have no easily measurable nitrogen or boron impurities. The other statements conflict with this: high nitrogen content would place the diamond in Type I rather than Type II; boron with negligible nitrogen points to Type IIb and would produce blue coloration, not the classic colorless field of Type IIa; and boron without nitrogen again aligns with IIb.

Type IIa diamonds are defined by an almost complete absence of impurities, especially nitrogen and boron. Because nitrogen causes certain absorption features that create color in many diamonds, its near absence makes Type IIa stones unusually colorless and often highly valued for their transparency. Boron is what gives Type IIb diamonds their blue hue and, in some cases, electrical conductivity; since Type IIa lacks boron as well, it remains colorless or nearly so.

So the best description is that they have no easily measurable nitrogen or boron impurities. The other statements conflict with this: high nitrogen content would place the diamond in Type I rather than Type II; boron with negligible nitrogen points to Type IIb and would produce blue coloration, not the classic colorless field of Type IIa; and boron without nitrogen again aligns with IIb.