Assignment 3 - Gems and their Physical Properties

Question 1

A mass of tiny, randomly
oriented crystals.

Answer 1

Aggregate

Question 2

An aggregate made up of individual crystals visible under
magnification.

Answer 2

Microcrystalline

Question 3

An aggregate made up of individual crystals detectable only under very high magnification.

Answer 3

Cryptocrystalline

Question 4

Change in a gem’s crystal
direction during or after growth.

Answer 4

Twinning

Question 5

A characteristic enclosed within a gemstone or reaching its
surface from the interior.

Answer 5

Inclusion

Question 6

Small pocket in a gem that’s filled with fluids and, sometimes, gas bubbles and tiny crystals.

Answer 6

Liquid inclusion

Question 7

A hollow cavity in a gem, usually filled with a liquid and a gas.

Answer 7

Two-phase inclusion

Question 8

A hollow cavity in a gem filled with a liquid, a gas, and one or more crystals.

Answer 8

Three-phase inclusion

Question 9

Atoms in a gem that aren’t part of its essential chemical composition.

Answer 9

Trace elements

Question 10

The characteristic external
crystal shape or form of a mineral.

Answer 10

Habit

Question 11

How heavy an object is in
relation to its size.

Answer 11

Density

Question 12

Ratio of the weight of a material to the weight of an equal volume of water.

Answer 12

Specific gravity (SG)

Question 13

A gemstone’s ability to
withstand wear, heat, and chemicals.

Answer 13

Durability

Question 14

How well a gemstone
resists scratches and abrasion.

Answer 14

Hardness

Question 15

How well a gemstone
resists breaking and chipping.

Answer 15

Toughness

Question 16

A smooth, flat break in a
gemstone parallel to planes of
atomic weakness, caused by weak or fewer bonds between atoms, or both.

Answer 16

Cleavage

Question 17

A flat break in a gemstone caused by concentrated included minerals parallel to a twinning plane.

Answer 17

Parting

Question 18

Any break in a gem other
than cleavage or parting.

Answer 18

Fracture

Question 19

A curved and ridged fracture in a gemstone, extending from the surface inward.

Answer 19

Conchoidal fracture

Question 20

How well a gemstone resists light, heat, and chemicals.

Answer 20

Stability

Question 21

Damage caused by sudden, extreme temperature changes.

Answer 21

Thermal shock

Question 22

The smallest group of atoms with both the characteristic
chemical composition and crystal structure of a mineral.

Answer 22

Unit cell

Question 23

How does a rough crystal get its start?

Answer 23

When the necessary chemical ingredients are available, and as the temperature cools, unit cells come together to form clusters. Each cluster is a template for
the fully grown crystal.

Question 24

How do inclusions and trace elements become part of a gem?

Answer 24

Inclusions are a byproduct of natural crystal growth. No gem grows in isolation, and almost every cut gem carries some trace of the different minerals it grew with.

As a crystal grows, some chemical elements that aren’t necessary for its growth can become trapped in its crystal structure. Since they’re present
at the atomic level, they’re not considered inclusions. They’re called trace elements, and they can affect a gem’s appearance in many ways.

Question 25

What features can help you recognize different rough gemstones?

Answer 25

Many rough gemstones have recognizable crystal shapes that canvhelp identify them.

Question 26

How does crystal shape affect the way a stone is cut?

Answer 26

The shape of the rough dictates the shape of the cut gem. Many gem crystals strongly influence the proportions of the faceted stones you’ll see in the colored stone trade. For instance, topaz crystals are often long and thin. To retain weight the lapidary might cut long, thin ovals or elongated rectangular step cuts.

Question 27

Why do different colored stones with similar dimensions have such different carat weights?

Answer 27

A gem’s density and specific gravity depend on the types of atoms it’s made of, and how closely they’re packed together.

Question 28

How do physical properties affect a gem’s durability?

Answer 28

The strength of a crystal is directly related to the strength of the bonds that bind its atoms together. A rough gem is exposed to external stress when it’s tumbled in a mountain stream and during mining operations; a finished gem, when it’s cut and polished, or during wear in jewelry. Depending on its crystal structure, a gem reacts to stress in different ways.

Question 29

What’s the difference between hardness and toughness?

Answer 29

Although hardness and toughness seem identical in everyday usage, in a gemological sense they have very different meanings. While hardness is resistance to scratching, toughness is resistance to breaking and chipping.

Question 30

What are the different ways a gem can break?

Answer 30

The different ways a gem breaks are called cleavage, parting, and fracture.

Question 31

The trace elements that cause corundum’s blue are A. iron and titanium. B. chromium and iron. C. aluminum and oxygen. D. aluminum and chromium.

Answer 31

A. iron and titanium.

Question 32

Many fashioned rubies have shallow proportions because they’re cut from A. flattened crystals. B. tall, columnar crystals. C. heavily included rough. D. flattened cleavage pieces.

Answer 32

A. flattened crystals.

Question 33

Which is classified in the orthorhombic crystal system? A. Beryl B. Topaz C. Garnet D. Corundum

Answer 33

B. Topaz

Question 34

Which gems are cryptocrystalline aggregates? A. Opal and amethyst B. Nephrite and jadeite C. Sugilite and lapis lazuli D. Chalcedony and turquoise

Answer 34

D. Chalcedony and turquoise

Question 35

Rough spinel often occurs as A. twinned crystals. B. tall, columnar crystals. C. microcrystalline aggregates. D. flattened cleavage fragments.

Answer 35

A. twinned crystals.

Question 36

Which element causes red in ruby and green in emerald? A. Calcium B. Potassium C. Chromium D. Magnesium

Answer 36

C. Chromium

Question 37

A unit cell defines a mineral’s A. size. B. habit. C. shape. D. basic identity.

Answer 37

D. basic identity.

Question 38

Needles of actinolite found in emerald are classified as A. cleavages. B. inclusions. C. penetration twins. D. impurity elements.

Answer 38

B. inclusions.

Question 39

Which type of twinning is caused by environmental change after the gem forms? A. Cyclic B. Primary C. Penetration D. Polysynthetic

Answer 39

D. Polysynthetic

Question 40

Which type of twinning looks as if two crystal halves are mirror images? A. Cyclic B. Contact C. Penetration D. Polysynthetic

Answer 40

B. Contact