Gem Identification & Grading Techniques Flashcards
7 cards from real CGA practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 7 Gem Identification & Grading Techniques flashcards as text
Which optical phenomenon causes a star sapphire to display a six-rayed star when viewed under a single light source?
Answer: Asterism
Asterism is caused by light reflecting off needle-like rutile inclusions oriented in multiple directions within the crystal.
A gemologist measures a stone's refractive index at 1.76 on a standard refractometer. Which gemstone does this value most closely match?
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.
When grading colored stones, which of the three color components has the greatest impact on a gem's value?
Answer: Saturation
Saturation (the intensity or purity of color) typically has the greatest influence on a colored gemstone's value.
Which inclusion type is most diagnostic for identifying natural emeralds versus synthetic emeralds grown by the hydrothermal method?
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.
A Chelsea color filter causes a synthetic blue spinel to appear red. What does this reaction indicate?
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.
Which characteristic best distinguishes a natural pearl from a cultured pearl when examined with X-ray radiography?
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.
What does a specific gravity value of approximately 3.52 indicate when identifying an unknown transparent colorless gemstone?
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.