GIA Gemstone Identification & Properties 2 — Questions and Answers
Question 1: Which optical phenomenon causes a six-rayed star to appear on the surface of a star sapphire?
- Adularescence
- Asterism (Correct answer)
- Chatoyancy
- Labradorescence
Correct answer: Asterism
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?
- 1.544–1.553
- 1.762–1.770 (Correct answer)
- 1.65–1.67
- 2.417
Correct answer: 1.762–1.770
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?
- Ruby
- Red spinel (Correct answer)
- Red garnet
- Rhodolite
Correct answer: Red spinel
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?
- Topaz has no cleavage; quartz has perfect cleavage
- Topaz has perfect basal cleavage; quartz has no cleavage (Correct answer)
- Both have identical cleavage in one direction
- Quartz has perfect cleavage in two directions; topaz has none
Correct answer: Topaz has perfect basal cleavage; quartz has no cleavage
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?
- 2.65
- 3.52 (Correct answer)
- 4.00
- 3.21
Correct answer: 3.52
Diamond has an SG of approximately 3.52, which results from its tightly packed carbon atoms in a cubic crystal structure.
Question 6: An alexandrite shows a strong color change from green in daylight to red under incandescent light. This effect is due to:
- Iron and titanium trace elements
- Chromium absorbing specific wavelengths of light (Correct answer)
- Vanadium causing double refraction
- Copper-induced pleochroism
Correct answer: Chromium absorbing specific wavelengths of light
Chromium in alexandrite absorbs light in a way that makes the gem appear green in daylight and red under incandescent (tungsten) light.
Question 7: Which testing method would BEST confirm whether a blue stone is aquamarine or blue topaz?
- Hardness test alone
- Specific gravity combined with refractive index (Correct answer)
- UV fluorescence alone
- Visual color inspection under daylight
Correct answer: Specific gravity combined with refractive index
Blue topaz (SG ~3.53, RI ~1.619–1.627) and aquamarine (SG ~2.72, RI ~1.577–1.583) differ significantly in both SG and RI, making combined testing definitive.
GIA Gemstone Identification & Properties 2
Which optical phenomenon causes a six-rayed star to appear on the surface of a star sapphire?