GIA Gemstone Identification & Properties 3 — Questions and Answers
Question 1: Which gemstone is composed of calcium carbonate (CaCO₃) and is therefore most susceptible to acid damage?
- Quartz
- Pearl (Correct answer)
- Tourmaline
- Spinel
Correct answer: Pearl
Pearls are composed of calcium carbonate (aragonite), which reacts with even mild acids, causing etching and surface damage.
Question 2: A gemstone has a hardness of 9 on the Mohs scale, a hexagonal crystal system, and can be red or blue. What is it?
- Spinel
- Corundum (Correct answer)
- Garnet
- Tourmaline
Correct answer: Corundum
Corundum (ruby and sapphire) has a Mohs hardness of 9, crystallizes in the hexagonal (trigonal) system, and occurs in colors including red (ruby) and blue (sapphire).
Question 3: Which phenomenon is responsible for the iridescent play-of-color seen in precious opal?
- Adularescence from orthoclase layers
- Diffraction of light by regular arrays of silica spheres (Correct answer)
- Interference in thin film surfaces
- Labradorescence from feldspar twinning
Correct answer: Diffraction of light by regular arrays of silica spheres
Precious opal's play-of-color results from light diffracting through a regular 3D grid of silica spheres of uniform size.
Question 4: What is pleochroism, and which gem is a classic example showing three different colors in three crystal directions?
- Play-of-color; opal
- Different colors in different viewing directions; tanzanite (Correct answer)
- Color zoning; watermelon tourmaline
- Fluorescence; ruby
Correct answer: Different colors in different viewing directions; tanzanite
Pleochroism is the display of different colors when viewed along different crystallographic axes; tanzanite (trichroic) shows blue, violet, and burgundy.
Question 5: A stone submitted for testing shows a weak Chelsea filter reaction, RI of ~1.72, and absorption bands in the red and blue. Which gem does this BEST describe?
- Synthetic ruby
- Tsavorite garnet (Correct answer)
- Demantoid garnet
- Green tourmaline
Correct answer: Tsavorite garnet
Tsavorite garnet (grossular) shows an RI near 1.72–1.74 and characteristic chromium-related absorption, with a weak-to-moderate Chelsea filter reaction.
Question 6: What term describes inclusions that look like a horse-tail in demantoid garnet, and what mineral forms them?
- Needle inclusions of rutile
- Horsetail inclusions of byssolite/chrysotile (Correct answer)
- Fingerprint inclusions of healed fractures
- Silk inclusions of sillimanite
Correct answer: Horsetail inclusions of byssolite/chrysotile
Demantoid garnet's signature horsetail inclusions are radiating fibers of byssolite (a type of chrysotile), and they are considered a positive ID feature.
Question 7: Which of the following describes the optic character 'doubly refractive' (DR)?
- Light travels at the same speed in all directions through the gem
- Light splits into two rays traveling at different speeds through the gem (Correct answer)
- The gem absorbs all wavelengths equally
- The gem shows no birefringence under polarized light
Correct answer: Light splits into two rays traveling at different speeds through the gem
In doubly refractive gems, an entering ray splits into two polarized rays traveling at different velocities, producing birefringence.
GIA Gemstone Identification & Properties 3
Which gemstone is composed of calcium carbonate (CaCO₃) and is therefore most susceptible to acid damage?