CGC Crystal Structure & Mineralogy

Question 1

Which crystal system is characterized by three axes of equal length meeting at right angles?

Accepted Answer

Cubic (Isometric)

Answer

The cubic (isometric) system has three equal axes (a1, a2, a3) all at 90° to each other, producing highly symmetrical forms like the cube and octahedron.

Question 2

On the Mohs hardness scale, which gemstone has a hardness of 9?

Accepted Answer

Corundum (Ruby/Sapphire)

Answer

Corundum, which includes ruby and sapphire, has a Mohs hardness of 9, second only to diamond (10) on the scale.

Question 3

What term describes the tendency of a mineral to break along flat planes related to its crystal structure?

Accepted Answer

Cleavage

Answer

Cleavage is the tendency of a mineral to break along smooth, flat planes that reflect the internal crystal structure and atomic bonding.

Question 4

Diamond belongs to which crystal system?

Accepted Answer

Cubic (Isometric)

Answer

Diamond crystallizes in the cubic (isometric) system, forming octahedral, cubic, and dodecahedral crystals due to the symmetric arrangement of carbon atoms.

Question 5

Which property describes the ratio of a gemstone's weight to the weight of an equal volume of water?

Accepted Answer

Specific Gravity

Answer

Specific gravity (SG) is the ratio of a mineral's density to that of water at 4°C, and it is a key diagnostic property for gemstone identification.

CGC - Certified Gemstone Consultant Practice Test

CGC - Certified Gemstone Consultant Practice Test

CGC Crystal Structure & Mineralogy