Assignment 3 - Gems and their Physical Properties Flashcards
• Describe the growth of a gem crystal. • Explain how and when inclusions become part of a gem. • Recognize the outward signs of a gem’s inner structure. • Understand the relationship between a gem’s structure and its finished shape. • Relate a gem’s physical properties to its usefulness in jewelry.
A mass of tiny, randomly
oriented crystals.
Aggregate
An aggregate made up of individual crystals visible under
magnification.
Microcrystalline
An aggregate made up of individual crystals detectable only under very high magnification.
Cryptocrystalline
Change in a gem’s crystal
direction during or after growth.
Twinning
A characteristic enclosed within a gemstone or reaching its
surface from the interior.
Inclusion
Small pocket in a gem that’s filled with fluids and, sometimes, gas bubbles and tiny crystals.
Liquid inclusion
A hollow cavity in a gem, usually filled with a liquid and a gas.
Two-phase inclusion
A hollow cavity in a gem filled with a liquid, a gas, and one or more crystals.
Three-phase inclusion
Atoms in a gem that aren’t part of its essential chemical composition.
Trace elements
The characteristic external
crystal shape or form of a mineral.
Habit
How heavy an object is in
relation to its size.
Density
Ratio of the weight of a material to the weight of an equal volume of water.
Specific gravity (SG)
A gemstone’s ability to
withstand wear, heat, and chemicals.
Durability
How well a gemstone
resists scratches and abrasion.
Hardness
How well a gemstone
resists breaking and chipping.
Toughness
A smooth, flat break in a
gemstone parallel to planes of
atomic weakness, caused by weak or fewer bonds between atoms, or both.
Cleavage
A flat break in a gemstone caused by concentrated included minerals parallel to a twinning plane.
Parting
Any break in a gem other
than cleavage or parting.
Fracture
A curved and ridged fracture in a gemstone, extending from the surface inward.
Conchoidal fracture
How well a gemstone resists light, heat, and chemicals.
Stability
Damage caused by sudden, extreme temperature changes.
Thermal shock
The smallest group of atoms with both the characteristic
chemical composition and crystal structure of a mineral.
Unit cell
How does a rough crystal get its start?
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.
How do inclusions and trace elements become part of a gem?
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.
What features can help you recognize different rough gemstones?
Many rough gemstones have
recognizable crystal shapes that canvhelp identify them.
How does crystal shape affect the way a stone is cut?
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.
Why do different colored stones with similar dimensions have such different carat weights?
A gem’s density and specific gravity depend on the types of atoms it’s made of, and how closely they’re packed together.
How do physical properties affect a gem’s durability?
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.
What’s the difference between hardness and toughness?
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.
What are the different ways a gem can break?
The different ways a gem breaks are called cleavage, parting, and fracture.
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.
A.
iron and titanium.
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.
A.
flattened crystals.
Which is classified in the orthorhombic crystal system?
A.
Beryl
B.
Topaz
C.
Garnet
D.
Corundum
B.
Topaz
Which gems are cryptocrystalline aggregates?
A.
Opal and amethyst
B.
Nephrite and jadeite
C.
Sugilite and lapis lazuli
D.
Chalcedony and turquoise
D.
Chalcedony and turquoise
Rough spinel often occurs as
A.
twinned crystals.
B.
tall, columnar crystals.
C.
microcrystalline aggregates.
D.
flattened cleavage fragments.
A.
twinned crystals.
Which element causes red in ruby and green in emerald?
A. Calcium
B. Potassium
C. Chromium
D. Magnesium
C. Chromium
A unit cell defines a mineral’s
A. size.
B. habit.
C. shape.
D. basic identity.
D. basic identity.
Needles of actinolite found in emerald are classified as
A. cleavages.
B. inclusions.
C. penetration twins.
D. impurity elements.
B. inclusions.
Which type of twinning is caused by environmental change after the gem forms?
A. Cyclic
B. Primary
C. Penetration
D. Polysynthetic
D. Polysynthetic
Which type of twinning looks as if two crystal halves are mirror images?
A. Cyclic
B. Contact
C. Penetration
D. Polysynthetic
B. Contact
