CGC Crystal Structure & Mineralogy 2

Question 1

Which crystal system does quartz belong to?

Accepted Answer

Trigonal

Answer

Quartz belongs to the trigonal crystal system (a subdivision of hexagonal), forming characteristic six-sided prisms with pyramidal terminations.

Question 2

What term describes two distinct minerals that have the same chemical composition but different crystal structures?

Accepted Answer

Polymorphs

Answer

Polymorphs are minerals with identical chemical compositions but different crystal structures; diamond and graphite are classic examples, both being pure carbon.

Question 3

Which type of luster is described as resembling the surface of a pearl or the inside of a shell?

Accepted Answer

Pearly

Answer

Pearly luster results from light reflection off internal cleavage planes and produces a soft, iridescent sheen similar to the surface of a pearl.

Question 4

Alexandrite is a variety of which mineral known for its dramatic color-change property?

Accepted Answer

Chrysoberyl

Answer

Alexandrite is a color-change variety of chrysoberyl (BeAl2O4) that appears green in daylight and red under incandescent light due to chromium absorption.

Question 5

What is the cleavage quality of diamond described as?

Accepted Answer

Perfect in four directions (octahedral)

Answer

Diamond has perfect octahedral cleavage in four directions parallel to the faces of the octahedron, which diamond cutters exploit when cleaving rough stones.

CGC - Certified Gemstone Consultant Practice Test

CGC - Certified Gemstone Consultant Practice Test

CGC Crystal Structure & Mineralogy 2