CDG Gemology & Mineralogy Fundamentals 3 — Questions and Answers
Question 1: Which of the following is a polymorph of diamond?
- Corundum
- Graphite (Correct answer)
- Quartz
- Spinel
Correct answer: Graphite
Graphite and diamond are both pure carbon polymorphs; graphite forms under low-pressure conditions while diamond requires extreme pressure.
Question 2: In gemology, 'dispersion' refers to which property of a gemstone?
- The separation of white light into spectral colors (Correct answer)
- The ability to resist scratching
- The bending of light at the surface
- The stone's ability to absorb certain wavelengths
Correct answer: The separation of white light into spectral colors
Dispersion is the difference in refractive index between the red and violet ends of the spectrum, producing the 'fire' seen in diamonds.
Question 3: Diamond's dispersion value is approximately:
- 0.013
- 0.044 (Correct answer)
- 0.071
- 0.156
Correct answer: 0.044
Diamond has a dispersion of 0.044 (B–G interval), which is relatively high and contributes to its pronounced fire.
Question 4: The term 'adamantine luster' is most closely associated with which mineral?
- Topaz
- Diamond (Correct answer)
- Sapphire
- Spinel
Correct answer: Diamond
Adamantine luster, the brightest luster type for non-metallic minerals, is named after the Greek word for diamond ('adamas').
Question 5: What causes Type IIb diamonds to be electrically semi-conductive?
- Nitrogen aggregates in platelets
- Hydrogen inclusions at grain boundaries
- Boron substituting for carbon in the lattice (Correct answer)
- Iron traces in the crystal structure
Correct answer: Boron substituting for carbon in the lattice
Type IIb diamonds contain boron impurities that replace carbon atoms, creating electron holes that allow limited electrical conductivity.
Question 6: A diamond crystal showing trigons on its octahedral face indicates:
- Post-growth acid etching
- Resorption or natural etching during ascent (Correct answer)
- Polishing marks from manufacturing
- Radiation exposure
Correct answer: Resorption or natural etching during ascent
Trigons are triangular etch pits formed by natural dissolution (resorption) as kimberlite magma rises toward the surface and conditions change.
Question 7: Which diamond type classification is defined by the absence of both nitrogen and boron impurities?
- Type Ia
- Type Ib
- Type IIa (Correct answer)
- Type IIb
Correct answer: Type IIa
Type IIa diamonds are chemically the purest, containing no detectable nitrogen or boron, and are often colorless with exceptional transparency.
CDG Gemology & Mineralogy Fundamentals 3
Which of the following is a polymorph of diamond?