CGA Gem Identification & Grading Techniques 3 — Questions and Answers
Question 1: Which spectroscopic absorption feature is diagnostic for identifying chromium as the cause of color in a ruby?
- Strong absorption at 415 nm (iron line)
- Doublet lines at 692 and 694 nm (Correct answer)
- Broad absorption in the blue region only
- Absorption at 450 nm (cobalt band)
Correct answer: Doublet lines at 692 and 694 nm
Chromium in ruby produces characteristic doublet lines at approximately 692 and 694 nm in the red region of the spectrum.
Question 2: A gemologist observes a weak orange-red fluorescence under longwave UV and no reaction under shortwave UV. Which stone is most likely being tested?
- Diamond (Type IIa)
- Natural ruby (Correct answer)
- Synthetic ruby (Verneuil)
- Alexandrite
Correct answer: Natural ruby
Natural rubies often show strong to moderate red or orange-red fluorescence under longwave UV, which can help distinguish them from some synthetic counterparts.
Question 3: When using a polariscope to test a gemstone, an 'ADR' (anomalous double refraction) reading most likely indicates which type of stone?
- A doubly refractive gemstone like tourmaline
- A singly refractive stone like garnet showing strain (Correct answer)
- A singly refractive stone like diamond
- An amorphous material like glass
Correct answer: A singly refractive stone like garnet showing strain
ADR occurs when a normally singly refractive (isotropic) stone, like a garnet or glass, shows strain birefringence producing irregular extinction patterns.
Question 4: Which gemstone treatment is most effectively detected by observing 'snow-globe' effect inclusions or feathers that reach the surface under magnification?
- Heat treatment
- Fracture filling with glass or resin (Correct answer)
- Irradiation
- Diffusion treatment
Correct answer: Fracture filling with glass or resin
Fracture-filled stones show characteristic flash effect, flow structures, and feathers reaching the surface where the filler has entered the stone.
Question 5: What is the primary purpose of using immersion microscopy when grading gemstones?
- To measure refractive index accurately
- To reduce surface reflections and improve visibility of internal features (Correct answer)
- To test hardness without damage
- To observe pleochroism more easily
Correct answer: To reduce surface reflections and improve visibility of internal features
Immersion in a liquid close to the stone's refractive index minimizes surface reflections, allowing clearer observation of internal inclusions and growth features.
Question 6: A transparent blue gemstone shows strong dichroism of blue and violet. Which mineral is the most likely candidate?
- Blue topaz
- Aquamarine
- Tanzanite (Correct answer)
- Blue tourmaline (indicolite)
Correct answer: Tanzanite
Tanzanite (blue zoisite) is famous for its strong trichroism showing blue, violet, and burgundy/red depending on the viewing direction.
Question 7: Which grading system component does 'VS1' refer to in the GIA diamond clarity grading scale?
- Very Slightly Included grade 1 — minor inclusions difficult to see under 10× magnification (Correct answer)
- Very Small grade 1 — referring to carat weight
- Visually Significant grade 1 — inclusions visible to the naked eye
- Very Superior grade 1 — near-flawless category
Correct answer: Very Slightly Included grade 1 — minor inclusions difficult to see under 10× magnification
VS1 (Very Slightly Included 1) denotes diamonds with minor inclusions that are difficult for a skilled grader to see under 10× magnification.
CGA Gem Identification & Grading Techniques 3
Which spectroscopic absorption feature is diagnostic for identifying chromium as the cause of color in a ruby?