GIA COLORED STONES ASSIGNMENT 19 OPAL
QUESTIONS AND ANSWERS 2024 – VERIFIED SOLUTIONS
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Opal Fundamentals & Composition (Questions 1-20)
1. What is the chemical composition of opal?
A) Silicon Dioxide (SiO₂)
B) Hydrated Silicon Dioxide (SiO₂·nH₂O)
C) Aluminum Oxide (Al₂O₃)
D) Sodium Aluminum Silicate
2. The play-of-color in opal is caused by:
A) Chemical impurities like chromium
B) Diffraction of light from a regular internal structure of silica spheres
C) Inclusions of foreign minerals
D) Iridescence from surface fractures
3. The water content in opal can range from:
A) 0% to 5%
B) 3% to 21%
C) 25% to 30%
D) It contains no water
4. What is the term for opal that does not show play-of-color?
A) Precious Opal
B) Potch Opal or Common Opal
C) Fire Opal
D) Crystal Opal
5. The specific gravity of opal typically falls between:
A) 1.98 and 2.05
B) 2.10 and 2.30
C) 1.99 and 2.25
D) 3.15 and 3.30
6. Opal's hardness on the Mohs scale is:
A) 5 to 6
B) 7 to 8
,C) 8 to 9
D) 6.5 to 7
7. The internal structure that causes play-of-color is made up of:
A) Randomly arranged silica grains
B) A grid of tiny rutile needles
C) Layers of silica and water
D) A three-dimensional lattice of uniform silica spheres
8. If the silica spheres in opal are all the same size, the play-of-color will be:
A) More vibrant and consist of a single color
B) Duller and consist of multiple colors
C) Non-existent
D) Only visible under ultraviolet light
9. Crazing in opal refers to:
A) Its brilliant luster
B) A fine network of cracks that can develop
C) Its color pattern
D) The process of cutting and polishing
10. Which of these is NOT a type of common opal?
A) Milky Opal
B) Pink Opal
C) Fire Opal
D) Moss Opal
11. The term "diaphaneity" for most precious opal is best described as:
A) Opaque
B) Transparent to Translucent
C) Metallic
D) Always transparent
12. What property allows opal to absorb and hold water?
A) Its crystalline structure
B) Its microporous nature
C) Its high specific gravity
D) Its chemical impurities
13. The refractive index of opal is closest to:
A) 1.43 - 1.46
, B) 1.52 - 1.54
C) 1.76 - 1.78
D) 2.41 - 2.42
14. The play-of-color effect is a form of:
A) Triboluminescence
B) Adularescence
C) Iridescence caused by diffraction
D) Aventurescence
15. Which size range of silica spheres will diffract red light?
A) The smallest spheres (~138 nm)
B) The largest spheres (~241 nm)
C) Medium-sized spheres (~180 nm)
D) The size of the spheres does not affect the color
16. What is the most critical factor in determining the value of a precious opal?
A) Its country of origin
B) Its size and carat weight
C) The bodycolor and play-of-color
D) Its hardness
17. The term "bodycolor" in opal refers to:
A) The background color against which the play-of-color is seen
B) The color of the brightest flash
C) The color of the opal when wet
D) The color of the potch layer
18. Opal is classified as:
A) A crystalline mineral
B) A mineraloid
C) An organic gem
D) A rock
19. Why is opal considered a delicate gemstone?
A) It is very rare
B) Its sensitivity to heat, pressure, and dehydration
C) It is always very small
D) It is easily scratched by dust (due to its 5-6 hardness)
QUESTIONS AND ANSWERS 2024 – VERIFIED SOLUTIONS
|| 100% GUARANTEED PASS <RECENT VERSION>
Opal Fundamentals & Composition (Questions 1-20)
1. What is the chemical composition of opal?
A) Silicon Dioxide (SiO₂)
B) Hydrated Silicon Dioxide (SiO₂·nH₂O)
C) Aluminum Oxide (Al₂O₃)
D) Sodium Aluminum Silicate
2. The play-of-color in opal is caused by:
A) Chemical impurities like chromium
B) Diffraction of light from a regular internal structure of silica spheres
C) Inclusions of foreign minerals
D) Iridescence from surface fractures
3. The water content in opal can range from:
A) 0% to 5%
B) 3% to 21%
C) 25% to 30%
D) It contains no water
4. What is the term for opal that does not show play-of-color?
A) Precious Opal
B) Potch Opal or Common Opal
C) Fire Opal
D) Crystal Opal
5. The specific gravity of opal typically falls between:
A) 1.98 and 2.05
B) 2.10 and 2.30
C) 1.99 and 2.25
D) 3.15 and 3.30
6. Opal's hardness on the Mohs scale is:
A) 5 to 6
B) 7 to 8
,C) 8 to 9
D) 6.5 to 7
7. The internal structure that causes play-of-color is made up of:
A) Randomly arranged silica grains
B) A grid of tiny rutile needles
C) Layers of silica and water
D) A three-dimensional lattice of uniform silica spheres
8. If the silica spheres in opal are all the same size, the play-of-color will be:
A) More vibrant and consist of a single color
B) Duller and consist of multiple colors
C) Non-existent
D) Only visible under ultraviolet light
9. Crazing in opal refers to:
A) Its brilliant luster
B) A fine network of cracks that can develop
C) Its color pattern
D) The process of cutting and polishing
10. Which of these is NOT a type of common opal?
A) Milky Opal
B) Pink Opal
C) Fire Opal
D) Moss Opal
11. The term "diaphaneity" for most precious opal is best described as:
A) Opaque
B) Transparent to Translucent
C) Metallic
D) Always transparent
12. What property allows opal to absorb and hold water?
A) Its crystalline structure
B) Its microporous nature
C) Its high specific gravity
D) Its chemical impurities
13. The refractive index of opal is closest to:
A) 1.43 - 1.46
, B) 1.52 - 1.54
C) 1.76 - 1.78
D) 2.41 - 2.42
14. The play-of-color effect is a form of:
A) Triboluminescence
B) Adularescence
C) Iridescence caused by diffraction
D) Aventurescence
15. Which size range of silica spheres will diffract red light?
A) The smallest spheres (~138 nm)
B) The largest spheres (~241 nm)
C) Medium-sized spheres (~180 nm)
D) The size of the spheres does not affect the color
16. What is the most critical factor in determining the value of a precious opal?
A) Its country of origin
B) Its size and carat weight
C) The bodycolor and play-of-color
D) Its hardness
17. The term "bodycolor" in opal refers to:
A) The background color against which the play-of-color is seen
B) The color of the brightest flash
C) The color of the opal when wet
D) The color of the potch layer
18. Opal is classified as:
A) A crystalline mineral
B) A mineraloid
C) An organic gem
D) A rock
19. Why is opal considered a delicate gemstone?
A) It is very rare
B) Its sensitivity to heat, pressure, and dehydration
C) It is always very small
D) It is easily scratched by dust (due to its 5-6 hardness)
