CGA Gem Identification & Grading Techniques — Questions and Answers
Question 1: What property is most commonly used to distinguish diamonds from simulants?
- Hardness
- Luster
- Refractive index (Correct answer)
- Color
Correct answer: Refractive index
The refractive index (RI) is a measure of how much light bends when it passes through a gemstone, and diamonds have a uniquely high RI compared to most simulants. This optical property is a critical diagnostic tool for gemologists, as it allows for precise identification and differentiation. While hardness and luster are also important, RI provides a more definitive distinction for diamonds.
Question 2: What is the primary scale used to measure gemstone hardness?
- Brinell Scale
- Mohs Scale (Correct answer)
- Vickers Scale
- Rockwell Scale
Correct answer: Mohs Scale
The Mohs Scale of Mineral Hardness, developed by Friedrich Mohs, is the primary scale used in gemology to measure a gemstone's resistance to scratching. It ranks minerals from 1 (softest, like talc) to 10 (hardest, like diamond) based on their ability to scratch softer materials. This scale is essential for identifying gems and understanding their durability for wear.
Question 3: What tool is typically used to examine internal inclusions in gems?
- Spectrometer
- Microscope
- 10x Loupe (Correct answer)
- Polariscope
Correct answer: 10x Loupe
A 10x loupe is a handheld magnifying glass that provides ten times magnification, making it the standard tool for examining internal inclusions and external blemishes in gems. Its portability and specific magnification are ideal for quickly assessing clarity characteristics that are crucial for grading and identification. While microscopes offer higher magnification, the 10x loupe is the primary and most commonly used tool for this purpose in the field.
Question 4: Which gemstone is typically treated with heat to enhance its color?
- Emerald
- Ruby
- Topaz
- Sapphire (Correct answer)
Correct answer: Sapphire
Sapphires are frequently treated with heat to enhance or change their color, making existing colors more vibrant or transforming pale stones into desirable hues. This heat treatment is a common and generally accepted practice in the gem industry to improve the gem's appearance and marketability. While other gems can also be treated, sapphire is particularly known for heat treatment for color enhancement.
Question 5: What does 'cut' refer to in the 4Cs of diamond grading?
- Shape
- Symmetry
- Brilliance
- Proportions and finish (Correct answer)
Correct answer: Proportions and finish
In the 4Cs of diamond grading (Cut, Color, Clarity, Carat Weight), 'cut' refers to how well a diamond's facets interact with light, which is determined by its proportions, symmetry, and polish (finish). A well-cut diamond maximizes its brilliance, fire, and scintillation, making it appear more lively and beautiful. It is distinct from the diamond's shape, which is its outline.
Question 6: Which term refers to a gemstone’s resistance to scratching?
- Toughness
- Hardness (Correct answer)
- Durability
- Luster
Correct answer: Hardness
Hardness in gemology specifically refers to a gemstone's resistance to scratching or abrasion, as measured by the Mohs Scale. This property is crucial for determining how well a gem will withstand daily wear and maintain its polished surface. Toughness, in contrast, refers to a gem's resistance to breaking or chipping.
Question 7: Which instrument is used to measure a gem’s refractive index?
- Spectroscope
- Refractometer (Correct answer)
- Polariscope
- Loupe
Correct answer: Refractometer
A refractometer is the specialized instrument used by gemologists to accurately measure a gemstone's refractive index (RI). By placing the gem on the instrument and observing the critical angle of light, the refractometer provides a precise reading that is vital for identifying gemstones and distinguishing them from simulants. This measurement is a fundamental part of gem identification.
Question 8: Which clarity characteristic can lower a diamond's value the most?
- Feathers
- Pinpoints
- Cavities
- Dark inclusions under the table (Correct answer)
Correct answer: Dark inclusions under the table
Dark inclusions, especially those visible under the table facet (the largest facet on the top of the diamond), significantly impact a diamond's beauty and value. They are easily seen and can disrupt the diamond's brilliance and transparency, making them a major clarity defect. While other inclusions can also reduce value, prominent dark inclusions in the most visible area are often the most detrimental.
Question 9: What is pleochroism in gemology?
- Light dispersion
- Color zoning
- Color change under lighting
- Color variation by viewing angle (Correct answer)
Correct answer: Color variation by viewing angle
Pleochroism is an optical phenomenon where a gemstone appears to be different colors when viewed from different crystallographic directions. This property is caused by the differential absorption of light as it passes through the crystal structure. Observing pleochroism with a dichroscope is a valuable diagnostic tool for identifying certain gemstones.
CGA Gem Identification & Grading Techniques
What property is most commonly used to distinguish diamonds from simulants?