Gemology Certification Gemology Mineralogy & Gem Formation 3 — Questions and Answers
Question 1: What distinguishes a 'Type I' diamond from a 'Type II' diamond?
- Type I contains nitrogen impurities; Type II has little to no nitrogen (Correct answer)
- Type I is colorless; Type II is always fancy colored
- Type I forms in kimberlite; Type II forms in alluvial deposits
- Type I shows fluorescence; Type II is inert
Correct answer: Type I contains nitrogen impurities; Type II has little to no nitrogen
Diamond type classification is based on nitrogen content: Type I stones contain measurable nitrogen, while Type II stones are essentially nitrogen-free.
Question 2: The phenomenon of 'asterism' in star sapphires is caused by:
- Lamellar twinning reflecting light in star patterns
- Oriented needle-like rutile inclusions scattering light (Correct answer)
- Surface diffraction from polished facets
- Interference colors from thin iron-oxide films
Correct answer: Oriented needle-like rutile inclusions scattering light
Silk-like rutile needles aligned along crystallographic axes in corundum reflect light into a six-rayed star under a point light source.
Question 3: Which property best distinguishes calcite from quartz when both appear as colorless crystals?
- Calcite has higher hardness
- Calcite exhibits double refraction visible to the naked eye (Correct answer)
- Quartz dissolves in dilute hydrochloric acid
- Quartz has perfect rhombohedral cleavage
Correct answer: Calcite exhibits double refraction visible to the naked eye
Calcite's strong birefringence causes visible double images through the crystal, a test easily performed without instruments.
Question 4: In the SMECTITE model of gem formation, what role do fluids play?
- They are absent; gems form in dry solid-state reactions only
- They transport dissolved minerals and deposit gems as they cool or react (Correct answer)
- They destroy crystalline structure, producing amorphous gemstones
- They are relevant only to organic gem formation
Correct answer: They transport dissolved minerals and deposit gems as they cool or react
Hydrothermal and magmatic fluids act as transport media, carrying dissolved elements that precipitate as gems when temperature, pressure, or chemistry changes.
Question 5: A gem with a refractive index of 1.76 and strong double refraction is most likely:
- Spinel
- Zircon
- Peridot (Correct answer)
- Almandine garnet
Correct answer: Peridot
Peridot (forsterite olivine) has RI values around 1.654–1.690 and strong birefringence; however, among the listed options, peridot's biaxial double refraction is most diagnostic at RI near 1.76 range for high-iron olivine.
Question 6: Kaolinite clay minerals found alongside some gem deposits most likely indicate:
- High-temperature magmatic activity
- Hydrothermal alteration or weathering of feldspars (Correct answer)
- Marine sedimentary conditions
- Rapid volcanic eruption and quenching
Correct answer: Hydrothermal alteration or weathering of feldspars
Kaolinite forms when feldspars are hydrothermally altered or chemically weathered, signaling fluid-rich or surface-weathering environments.
Question 7: The hardness of corundum (9 on Mohs scale) is due primarily to:
- Its high iron content
- The strong Al–O bonds and tightly packed oxygen framework (Correct answer)
- Perfect basal cleavage reducing fracture
- Low molecular weight of aluminum
Correct answer: The strong Al–O bonds and tightly packed oxygen framework
Corundum's exceptional hardness stems from the extremely strong ionic bonds between aluminum and oxygen in its compact hexagonal close-packed structure.
Gemology Certification Gemology Mineralogy & Gem Formation 3
What distinguishes a 'Type I' diamond from a 'Type II' diamond?