CGC Crystal Structure & Mineralogy 3

Question 1

What is the term for the phenomenon where a mineral displays different colors when viewed along different crystallographic axes?

Accepted Answer

Pleochroism

Answer

Pleochroism is the optical property of absorbing different wavelengths of light along different crystal axes, causing the mineral to show different colors depending on viewing direction.

Question 2

Spinel crystallizes in the cubic system and commonly forms which crystal habit?

Accepted Answer

Octahedra

Answer

Spinel characteristically crystallizes as octahedra, often as well-formed single crystals, and twinned octahedral crystals (spinel twins) are named after the mineral.

Question 3

Which silicate structural group do garnets belong to?

Accepted Answer

Nesosilicates (isolated tetrahedra)

Answer

Garnets are nesosilicates, meaning their structure consists of isolated SiO4 tetrahedra linked together by metal cations rather than sharing oxygen atoms with adjacent tetrahedra.

Question 4

What causes the adularescence (moonstone glow) in orthoclase feldspar?

Accepted Answer

Light scattering from alternating albite and orthoclase layers

Answer

Adularescence results from light scattering within submicroscopic alternating layers of orthoclase and albite feldspar, which create a soft billowing blue-white glow.

Question 5

A gemstone with a specific gravity of 3.52 and Mohs hardness of 10 is most likely which mineral?

Accepted Answer

Diamond

Answer

Diamond has a specific gravity of approximately 3.52 and a Mohs hardness of 10, making this combination a definitive diagnostic combination for diamond.

CGC - Certified Gemstone Consultant Practice Test

CGC - Certified Gemstone Consultant Practice Test

CGC Crystal Structure & Mineralogy 3