GIA Gemstone Identification & Properties 3

Question 1

Which gemstone is composed of calcium carbonate (CaCO₃) and is therefore most susceptible to acid damage?

Accepted Answer

Pearl

Answer

Pearls are composed of calcium carbonate (aragonite), which reacts with even mild acids, causing etching and surface damage.

Question 2

A gemstone has a hardness of 9 on the Mohs scale, a hexagonal crystal system, and can be red or blue. What is it?

Accepted Answer

Corundum

Answer

Corundum (ruby and sapphire) has a Mohs hardness of 9, crystallizes in the hexagonal (trigonal) system, and occurs in colors including red (ruby) and blue (sapphire).

Question 3

Which phenomenon is responsible for the iridescent play-of-color seen in precious opal?

Accepted Answer

Diffraction of light by regular arrays of silica spheres

Answer

Precious opal's play-of-color results from light diffracting through a regular 3D grid of silica spheres of uniform size.

Question 4

What is pleochroism, and which gem is a classic example showing three different colors in three crystal directions?

Accepted Answer

Different colors in different viewing directions; tanzanite

Answer

Pleochroism is the display of different colors when viewed along different crystallographic axes; tanzanite (trichroic) shows blue, violet, and burgundy.

Question 5

A stone submitted for testing shows a weak Chelsea filter reaction, RI of ~1.72, and absorption bands in the red and blue. Which gem does this BEST describe?

Accepted Answer

Tsavorite garnet

Answer

Tsavorite garnet (grossular) shows an RI near 1.72–1.74 and characteristic chromium-related absorption, with a weak-to-moderate Chelsea filter reaction.

(GIA) Certified Diamonds Gemological Institute of America Practice Test

(GIA) Certified Diamonds Gemological Institute of America Practice Test

GIA Gemstone Identification & Properties 3