CGA CGA Diamond Grading & Gemological Science 2 — Questions and Answers
Question 1: A gemologist uses a spectroscope to identify a gemstone. What does this instrument measure?
- Refractive index
- Specific gravity
- Absorption of light wavelengths (Correct answer)
- Thermal conductivity
Correct answer: Absorption of light wavelengths
A spectroscope measures the specific wavelengths of light absorbed by a gemstone, producing an absorption spectrum that is unique to each gem variety and aids in identification.
Question 2: What is the refractive index (RI) of a diamond?
- 1.52
- 1.65
- 1.76
- 2.42 (Correct answer)
Correct answer: 2.42
Diamond has an exceptionally high refractive index of 2.42, which is one reason it has such extraordinary brilliance and fire compared to other gemstones.
Question 3: What is 'dispersion' in gemology?
- The scattering of inclusions within a stone
- The separation of white light into spectral colors (Correct answer)
- The distribution of color throughout a stone
- The spread of a gem's weight across its surface
Correct answer: The separation of white light into spectral colors
Dispersion is the ability of a gemstone to separate white light into its spectral colors, which in diamonds produces the flashes of color known as 'fire.'
Question 4: Which gemological instrument is used to measure a gemstone's specific gravity?
- Refractometer
- Hydrostatic balance (Correct answer)
- Polariscope
- Chelsea filter
Correct answer: Hydrostatic balance
A hydrostatic balance measures specific gravity by comparing a gemstone's weight in air to its weight when submerged in water, using Archimedes' principle.
Question 5: In diamond grading, what is a 'feather'?
- A surface scratch
- A type of inclusion that appears as a small fracture or crack (Correct answer)
- A cloud-like group of pinpoints
- A growth irregularity in the crystal structure
Correct answer: A type of inclusion that appears as a small fracture or crack
A feather is a small fracture or break within a diamond that resembles a feather when viewed under magnification, and its size and location affect the clarity grade.
Question 6: What is the Mohs hardness rating of diamond?
- 8
- 9
- 9.5
- 10 (Correct answer)
Correct answer: 10
Diamond rates 10 on the Mohs hardness scale, making it the hardest natural substance known, though it can still cleave along crystallographic planes.
CGA CGA Diamond Grading & Gemological Science 2
A gemologist uses a spectroscope to identify a gemstone.
What does this instrument measure?