CGA Gem Identification & Grading Techniques 2

Question 1

Which optical phenomenon causes a star sapphire to display a six-rayed star when viewed under a single light source?

Accepted Answer

Asterism

Answer

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?

Accepted Answer

Ruby/Corundum (1.76–1.77)

Answer

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?

Accepted Answer

Saturation

Answer

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?

Accepted Answer

Three-phase inclusions

Answer

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?

Accepted Answer

The stone contains chromium as a colorant

Answer

The Chelsea filter isolates red and yellow-green wavelengths; chromium-colored stones transmit red, causing them to appear red through the filter.

(CGA) Certified Gemologist Appraiser Practice Test

(CGA) Certified Gemologist Appraiser Practice Test

CGA Gem Identification & Grading Techniques 2