CGA Gem Identification & Grading Techniques 3

Question 1

Which spectroscopic absorption feature is diagnostic for identifying chromium as the cause of color in a ruby?

Accepted Answer

Doublet lines at 692 and 694 nm

Answer

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?

Accepted Answer

Natural ruby

Answer

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?

Accepted Answer

A singly refractive stone like garnet showing strain

Answer

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?

Accepted Answer

Fracture filling with glass or resin

Answer

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?

Accepted Answer

To reduce surface reflections and improve visibility of internal features

Answer

Immersion in a liquid close to the stone's refractive index minimizes surface reflections, allowing clearer observation of internal inclusions and growth features.

(CGA) Certified Gemologist Appraiser Practice Test

(CGA) Certified Gemologist Appraiser Practice Test

CGA Gem Identification & Grading Techniques 3