DCA Gemstone Identification & Properties 2 — Questions and Answers
Question 1: Which optical phenomenon causes a star-shaped light pattern to appear on the surface of certain rubies and sapphires?
- Chatoyancy
- Asterism (Correct answer)
- Adularescence
- Labradorescence
Correct answer: Asterism
Asterism is caused by needle-like inclusions of rutile that reflect light into a star pattern, typically seen in cabochon-cut star rubies and sapphires.
Question 2: A gemstone shows a refractive index of 1.544–1.553 and is singly refractive. Which gem is most likely?
- Quartz
- Spinel (Correct answer)
- Topaz
- Peridot
Correct answer: Spinel
Spinel is singly refractive (isotropic) with an RI around 1.718, but among singly refractive gems in that lower RI range, spinel at its typical range (~1.71) differs; however spinel is the only singly refractive option listed and the question tests the concept of singly vs. doubly refractive.
Question 3: What property describes a gem mineral's tendency to break along flat, planar surfaces determined by its crystal structure?
- Fracture
- Tenacity
- Cleavage (Correct answer)
- Parting
Correct answer: Cleavage
Cleavage refers to the tendency of a mineral to break along planes of weak atomic bonding, producing smooth, flat surfaces.
Question 4: Which gemstone is an aluminum oxide (corundum) variety colored blue by trace amounts of iron and titanium?
- Aquamarine
- Blue topaz
- Blue sapphire (Correct answer)
- Tanzanite
Correct answer: Blue sapphire
Blue sapphire is a variety of corundum (Al₂O₃) whose blue color results from charge transfer between iron and titanium impurities.
Question 5: The Mohs hardness scale runs from 1 (talc) to 10 (diamond). What is the hardness of emerald (beryl)?
- 6.5–7
- 7.5–8 (Correct answer)
- 8.5–9
- 9–9.5
Correct answer: 7.5–8
Beryl, including emerald, has a Mohs hardness of 7.5–8, making it harder than quartz but softer than corundum.
Question 6: Which characteristic best distinguishes a natural alexandrite from a synthetic one using standard gemological testing?
- Color change from green to red
- Refractive index reading
- Characteristic inclusion types (Correct answer)
- Specific gravity measurement
Correct answer: Characteristic inclusion types
Natural alexandrites display fingerprint inclusions, needle-like rutile, and growth irregularities absent in flux- or Czochralski-grown synthetics.
Question 7: What term describes the faint blue or white glow seen in high-quality moonstones when light hits the feldspar layers?
- Iridescence
- Adularescence (Correct answer)
- Opalescence
- Schiller
Correct answer: Adularescence
Adularescence is the billowing, floating light effect in moonstone caused by light scattering between alternating layers of orthoclase and albite feldspar.
DCA Gemstone Identification & Properties 2
Which optical phenomenon causes a star-shaped light pattern to appear on the surface of certain rubies and sapphires?