DCA Gemstone Identification & Properties 3 — Questions and Answers
Question 1: A customer brings in a green stone that is strongly trichroic, showing blue, yellow, and red through a dichroscope. Which gem is this most likely?
- Tsavorite garnet
- Alexandrite
- Tourmaline
- Andalusite (Correct answer)
Correct answer: Andalusite
Andalusite is a strongly trichroic gem showing green, orange-red, and yellow-brown colors, making it one of the most diagnostic trichroic stones.
Question 2: Which of the following gems is NOT a member of the garnet group?
- Tsavorite
- Demantoid
- Tanzanite (Correct answer)
- Spessartite
Correct answer: Tanzanite
Tanzanite is a blue-violet variety of the mineral zoisite, while tsavorite, demantoid, and spessartite are all garnet group members.
Question 3: What does specific gravity (SG) measure in gemology?
- A gem's resistance to scratching
- The ratio of a gem's weight to the weight of an equal volume of water (Correct answer)
- The speed at which light travels through a gem
- A gem's ability to conduct heat
Correct answer: The ratio of a gem's weight to the weight of an equal volume of water
Specific gravity is the ratio of a gem's density to that of water at 4°C, serving as a useful identification tool.
Question 4: Which phenomenon causes labradorite to display rainbow flashes of color due to light interference between twinned feldspar layers?
- Adularescence
- Chatoyancy
- Labradorescence (Correct answer)
- Iridescence
Correct answer: Labradorescence
Labradorescence is the vivid spectral play of color in labradorite caused by light interference between alternating structural layers.
Question 5: A gem tests singly refractive, has an SG of 3.52, and rates 10 on Mohs hardness. What is it?
- Moissanite
- White sapphire
- Diamond (Correct answer)
- White spinel
Correct answer: Diamond
Diamond is the only gem with all three properties: singly refractive (cubic), SG 3.52, and Mohs hardness 10.
Question 6: Which gemstone is the birthstone for May and gets its characteristic green color primarily from chromium?
- Peridot
- Tsavorite garnet
- Emerald (Correct answer)
- Green tourmaline
Correct answer: Emerald
Emerald, the May birthstone, owes its deep green color to trace amounts of chromium (and sometimes vanadium) within the beryl crystal lattice.
Question 7: When a gemologist observes strong doubling of back facets through the table of a stone, which gem property is being detected?
- High refractive index
- Strong birefringence (Correct answer)
- Low dispersion
- Fluorescence
Correct answer: Strong birefringence
Strong birefringence (double refraction) causes facet edges and inclusions to appear doubled when viewed through the gem, as in zircon and peridot.
DCA Gemstone Identification & Properties 3
A customer brings in a green stone that is strongly trichroic, showing blue, yellow, and red through a dichroscope.
Which gem is this most likely?