Type IIb diamonds are characterized by...

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

Answer: (C)

Impurities determine diamond type and its color and electrical behavior. In Type IIb diamonds, boron atoms substitute for carbon in the crystal lattice. This boron incorporation gives the diamond its blue to gray color and makes it electrically conductive. Crucially, there are negligible nitrogen impurities in Type IIb stones, distinguishing them from nitrogen-rich diamond types. Therefore, the best description is that Type IIb diamonds contain boron substituting for carbon with negligible nitrogen impurities. The other options fail because they either imply no impurities, or a high nitrogen content, or omit the essential detail about the negligible nitrogen presence alongside boron.