Gemology Certification Gemology Mineralogy & Gem Formation 2 — Questions and Answers
Question 1: Which crystal system is characterized by three axes of unequal length, all meeting at right angles?
- Orthorhombic (Correct answer)
- Triclinic
- Monoclinic
- Trigonal
Correct answer: Orthorhombic
The orthorhombic system has three mutually perpendicular axes (a, b, c) all at 90° but all of different lengths.
Question 2: Pegmatite deposits are significant in gemology primarily because they:
- Form at very high pressures deep in the mantle
- Allow slow crystallization that produces exceptionally large crystals (Correct answer)
- Occur exclusively in sedimentary basins
- Are the primary source of corundum gemstones
Correct answer: Allow slow crystallization that produces exceptionally large crystals
Pegmatites cool and crystallize slowly from volatile-rich magmatic fluids, enabling growth of unusually large gem crystals such as tourmaline and beryl.
Question 3: The term 'specific gravity' in gemology refers to:
- The weight of a gem per unit surface area
- The ratio of a gem's density to the density of water at 4°C (Correct answer)
- The force exerted by gravity on the gem
- The refractive index multiplied by density
Correct answer: The ratio of a gem's density to the density of water at 4°C
Specific gravity (SG) is a dimensionless ratio comparing the gem's density to that of pure water at 4°C.
Question 4: Which metamorphic process is most responsible for the formation of gem-quality garnets?
- Contact metamorphism at low pressure
- Regional metamorphism at high pressure and temperature (Correct answer)
- Hydrothermal alteration of sandstone
- Diagenesis of carbonate sediments
Correct answer: Regional metamorphism at high pressure and temperature
Regional metamorphism subjects rocks to the high pressures and temperatures needed to stabilize the dense garnet crystal structure.
Question 5: A mineral's 'cleavage' is determined by:
- The hardness of the mineral along different axes
- Planes of weakness parallel to atomic bonding directions (Correct answer)
- The shape of crystal faces that formed during growth
- The angle at which light refracts through the crystal
Correct answer: Planes of weakness parallel to atomic bonding directions
Cleavage follows planes where atomic bonds are weakest, causing the mineral to break smoothly along those directions.
Question 6: Alexandrite's color-change phenomenon is caused by:
- Interference of light in thin surface layers
- Chromium absorbing light differently under incandescent vs. daylight (Correct answer)
- Iron causing fluorescence under UV only
- Structural color from diffraction gratings
Correct answer: Chromium absorbing light differently under incandescent vs. daylight
Chromium in alexandrite transmits both red and green wavelengths in a ratio that appears green in daylight but red under incandescent light.
Question 7: Which environment of formation is most associated with gem-quality spinel?
- Granitic pegmatites
- Metamorphic marble deposits (Correct answer)
- Basaltic volcanic pipes
- Evaporite sedimentary sequences
Correct answer: Metamorphic marble deposits
Gem spinel typically forms in contact-metamorphosed limestone (marble) environments alongside ruby, particularly in Myanmar and Sri Lanka.
Gemology Certification Gemology Mineralogy & Gem Formation 2
Which crystal system is characterized by three axes of unequal length, all meeting at right angles?