GIA Gemstone Identification & Properties 2

Question 1

Which optical phenomenon causes a six-rayed star to appear on the surface of a star sapphire?

Accepted Answer

Asterism

Answer

Asterism is caused by light reflecting off needle-like rutile inclusions arranged in intersecting planes within the gem.

Question 2

What is the refractive index (RI) range used to distinguish corundum (ruby/sapphire) from other red or blue gemstones?

Accepted Answer

1.762–1.770

Answer

Corundum has an RI of approximately 1.762–1.770, which clearly separates it from glass and lower-RI simulants.

Question 3

A gemstone displays strong red fluorescence under long-wave UV and shows a single refraction. Which gem is most consistent with these properties?

Accepted Answer

Red spinel

Answer

Red spinel is singly refractive (isotropic) and commonly shows strong red fluorescence under long-wave UV, unlike doubly refractive ruby.

Question 4

What cleavage characteristic is most diagnostic when differentiating topaz from quartz?

Accepted Answer

Topaz has perfect basal cleavage; quartz has no cleavage

Answer

Topaz has perfect basal cleavage in one direction, while quartz has no cleavage, making this a key identification feature.

Question 5

Which specific gravity (SG) value is most consistent with a diamond?

Accepted Answer

3.52

Answer

Diamond has an SG of approximately 3.52, which results from its tightly packed carbon atoms in a cubic crystal structure.

(GIA) Certified Diamonds Gemological Institute of America Practice Test