CGA Gem Identification & Grading Techniques 4 — Questions and Answers
Question 1: Which treatment applied to rubies and sapphires is most commonly detected by the presence of flux residues, rounded crystal inclusions, and healed fractures under magnification?
- Beryllium diffusion
- Glass fracture filling
- High-temperature heat treatment with flux (Correct answer)
- Irradiation
Correct answer: High-temperature heat treatment with flux
Flux-assisted heat treatment leaves distinctive flux residues, healed feathers, and altered inclusion appearances visible under magnification.
Question 2: In colored stone grading, what does the term 'extinction' refer to?
- Loss of color when a stone is viewed in low light
- Dark areas within the stone where no color is visible due to light leakage (Correct answer)
- The process of removing surface scratches
- Complete absorption of one wavelength in a dichroic stone
Correct answer: Dark areas within the stone where no color is visible due to light leakage
Extinction refers to dark or black areas in a faceted colored stone caused by light leaking out through the pavilion rather than reflecting back to the viewer.
Question 3: A gemologist encounters a stone with observed doubling of back facets under magnification and a refractive index reading above the refractometer's limit (over 1.81). Which gemstone is most likely?
- Zircon (Correct answer)
- Diamond
- Synthetic moissanite
- Spessartite garnet
Correct answer: Zircon
High-type zircon has refractive indices of 1.925–1.984 (above most refractometer limits) and strong birefringence causing visible doubling of back facets.
Question 4: Which method is the most reliable for detecting beryllium diffusion treatment in sapphires?
- Standard refractometer testing
- Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) (Correct answer)
- Chelsea color filter examination
- Standard UV fluorescence testing
Correct answer: Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS)
LA-ICP-MS can detect trace amounts of beryllium and its concentration profile within the stone, which reveals diffusion treatment that other standard tests cannot identify.
Question 5: What is the significance of 'graining' observed in diamonds during clarity grading?
- Surface scratches from wear
- Internal growth irregularities related to the crystal's formation (Correct answer)
- Polishing marks left by the cutter
- Fractures filled with foreign material
Correct answer: Internal growth irregularities related to the crystal's formation
Graining in diamonds refers to irregular crystal growth patterns that appear as faint lines, curves, or planes, and can affect clarity and transparency.
Question 6: A colorless stone tests negative on the diamond thermal tester but shows a single refraction reading and specific gravity of 3.17. Which simulant is most likely?
- Cubic zirconia (SG 5.60–5.90)
- Synthetic moissanite (SG ~3.22) (Correct answer)
- Strontium titanate (SG ~5.13)
- YAG — yttrium aluminum garnet (SG ~4.55)
Correct answer: Synthetic moissanite (SG ~3.22)
Synthetic moissanite has SG ~3.22 and is doubly refractive (not single), but closely matches diamond in thermal conductivity; a SG near 3.17 combined with these features suggests moissanite testing.
Question 7: Which gemstone property is defined as the resistance to scratching and is expressed on the Mohs scale?
- Toughness
- Tenacity
- Hardness (Correct answer)
- Cleavage
Correct answer: Hardness
Hardness on the Mohs scale measures a mineral's resistance to scratching, ranging from 1 (talc) to 10 (diamond).
CGA Gem Identification & Grading Techniques 4
Which treatment applied to rubies and sapphires is most commonly detected by the presence of flux residues, rounded crystal inclusions, and healed fractures under magnification?