DCA Gemstone Identification & Properties 3

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?

Accepted Answer

Andalusite

Answer

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?

Accepted Answer

Tanzanite

Answer

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?

Accepted Answer

The ratio of a gem's weight to the weight of an equal volume of water

Answer

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?

Accepted Answer

Labradorescence

Answer

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?

Accepted Answer

Diamond

Answer

Diamond is the only gem with all three properties: singly refractive (cubic), SG 3.52, and Mohs hardness 10.

(DCA) Certified Diamond Council of America Diamond Specialist Practice Test