CGC Gemstone Identification & Classification 2

Question 1

A colorless stone reads RI 1.544–1.553, birefringence 0.009, and SG 2.65. What is it most likely?

Accepted Answer

Quartz

Answer

These RI, birefringence, and SG values are diagnostic for quartz.

Question 2

Garnets crystallize in which crystal system, making them singly refractive?

Accepted Answer

Isometric (cubic)

Answer

Garnets are isometric, so light travels at one speed and they are singly refractive.

Question 3

Under a polariscope, a stone stays dark through a full 360° rotation. What does this usually indicate?

Accepted Answer

Singly refractive or amorphous

Answer

Remaining dark in all positions indicates a singly refractive or amorphous material.

Question 4

Which instrument is used specifically to observe pleochroism in a gemstone?

Accepted Answer

Dichroscope

Answer

A dichroscope shows two pleochroic colors side by side in doubly refractive stones.

Question 5

Which gem gives an over-the-limit (OTL) reading on a standard refractometer?

Accepted Answer

Cubic zirconia

Answer

Cubic zirconia's RI of about 2.15 exceeds the standard 1.81 contact-liquid limit.

CGC - Certified Gemstone Consultant Practice Test