DCA Laboratory Testing & Equipment 3 — Questions and Answers
Question 1: Which gemological microscope illumination technique is best for observing surface blemishes such as scratches and polish lines?
- Darkfield illumination
- Brightfield illumination
- Oblique illumination (Correct answer)
- Fiber-optic spot illumination
Correct answer: Oblique illumination
Oblique (or side) illumination rakes light across the surface at a low angle, making surface irregularities highly visible by casting micro-shadows.
Question 2: What is the primary advantage of using immersion in a liquid-filled cell when examining a diamond under a microscope?
- It enhances fluorescence
- It reduces surface reflections and improves clarity observation (Correct answer)
- It increases magnification
- It measures refractive index directly
Correct answer: It reduces surface reflections and improves clarity observation
Immersion in a liquid with a refractive index close to the stone's reduces internal reflections, making it easier to observe internal features.
Question 3: An FTIR (Fourier Transform Infrared) spectrometer is most useful for identifying diamonds because it can:
- Measure carat weight precisely
- Detect nitrogen aggregation patterns to classify diamond type (Correct answer)
- Map surface topography
- Distinguish cut grades
Correct answer: Detect nitrogen aggregation patterns to classify diamond type
FTIR analysis reveals how nitrogen atoms are arranged (Type Ia, Ib, IIa, IIb), which also helps identify treatments and synthetics.
Question 4: Cathodoluminescence (CL) imaging helps gemologists identify HPHT-treated diamonds because treated stones often show:
- A blue haze under CL
- Unusual growth sector patterns different from natural diamonds (Correct answer)
- Increased birefringence
- Complete absence of luminescence
Correct answer: Unusual growth sector patterns different from natural diamonds
HPHT treatment alters the diamond's lattice strain and growth sectors, producing distinctive CL patterns not seen in untreated natural diamonds.
Question 5: Which statement about using a refractometer to test a mounted diamond is correct?
- It gives an accurate RI reading from the table facet
- It cannot be used because diamond's RI exceeds the instrument's range (Correct answer)
- It works only in shortwave UV light
- It requires immersion oil with RI of 1.81
Correct answer: It cannot be used because diamond's RI exceeds the instrument's range
Diamond's refractive index (~2.42) exceeds the upper limit (~1.81) of standard gemological refractometers, so no reading is obtainable.
Question 6: A spectroscope is used in diamond grading labs primarily to:
- Determine exact carat weight
- Identify absorption lines indicating treatments or fancy color origins (Correct answer)
- Measure fluorescence intensity
- Assess symmetry of facet arrangement
Correct answer: Identify absorption lines indicating treatments or fancy color origins
Absorption spectra reveal characteristic lines (e.g., the 415 nm line for Cape series diamonds) that indicate nitrogen presence, color origin, and treatment history.
Question 7: In a standard gemological lab, which tool would FIRST be used when a new diamond sample arrives for identification?
- Polariscope
- Thermal conductivity probe (Correct answer)
- FTIR spectrometer
- Scanning electron microscope
Correct answer: Thermal conductivity probe
The thermal conductivity probe is a quick, non-destructive, and inexpensive first step that confirms whether the stone is diamond before more advanced testing.
DCA Laboratory Testing & Equipment 3
Which gemological microscope illumination technique is best for observing surface blemishes such as scratches and polish lines?