CGA Gem Identification & Grading Techniques 2 — Questions and Answers
Question 1: Which optical phenomenon causes a star sapphire to display a six-rayed star when viewed under a single light source?
- Adularescence
- Asterism (Correct answer)
- Chatoyancy
- Labradorescence
Correct answer: Asterism
Asterism is caused by light reflecting off needle-like rutile inclusions oriented in multiple directions within the crystal.
Question 2: A gemologist measures a stone's refractive index at 1.76 on a standard refractometer. Which gemstone does this value most closely match?
- Quartz (1.54)
- Spinel (1.72)
- Ruby/Corundum (1.76–1.77) (Correct answer)
- Diamond (2.42)
Correct answer: Ruby/Corundum (1.76–1.77)
Ruby and sapphire (corundum) have refractive indices of approximately 1.762–1.770, making 1.76 a classic corundum reading.
Question 3: When grading colored stones, which of the three color components has the greatest impact on a gem's value?
- Hue
- Tone
- Saturation (Correct answer)
- Distribution
Correct answer: Saturation
Saturation (the intensity or purity of color) typically has the greatest influence on a colored gemstone's value.
Question 4: Which inclusion type is most diagnostic for identifying natural emeralds versus synthetic emeralds grown by the hydrothermal method?
- Fingerprint inclusions
- Three-phase inclusions (Correct answer)
- Chevron growth patterns
- Nail-head spicules
Correct answer: Three-phase inclusions
Three-phase inclusions (containing liquid, gas, and a solid crystal) are characteristic of natural Colombian emeralds and are absent in most synthetics.
Question 5: A Chelsea color filter causes a synthetic blue spinel to appear red. What does this reaction indicate?
- The stone contains chromium as a colorant (Correct answer)
- The stone is colored by iron
- The stone has strong fluorescence
- The stone is heat-treated
Correct answer: The stone contains chromium as a colorant
The Chelsea filter isolates red and yellow-green wavelengths; chromium-colored stones transmit red, causing them to appear red through the filter.
Question 6: Which characteristic best distinguishes a natural pearl from a cultured pearl when examined with X-ray radiography?
- Size of the pearl
- Presence of a bead nucleus in cultured pearls (Correct answer)
- Surface luster differences
- Weight discrepancy
Correct answer: Presence of a bead nucleus in cultured pearls
X-ray radiography reveals the shell-bead nucleus inside cultured pearls, which is absent in natural pearls consisting entirely of nacre layers.
Question 7: What does a specific gravity value of approximately 3.52 indicate when identifying an unknown transparent colorless gemstone?
- White topaz
- White sapphire
- Diamond (Correct answer)
- Goshenite (colorless beryl)
Correct answer: Diamond
Diamond has a specific gravity of 3.52, which is one of its key identifying constants along with its refractive index of 2.42.
CGA Gem Identification & Grading Techniques 2
Which optical phenomenon causes a star sapphire to display a six-rayed star when viewed under a single light source?