Key Concepts and Terms Flashcards
A mass of tiny, randomly oriented crystals.
Aggregate
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 curved and ridged fracture in a gemstone, extending from the surface inward.
Conchoidal fracture
An aggregate made up of individual crystals detectable only under very high magnification.
Cryptocrystalline
How heavy an object is in relation to its size.
Density
A gemstone’s ability to withstand wear, heat, and chemicals.
Durability
Any break in a gem other than cleavage or parting.
Fracture
The characteristic external crystal shape or form of a mineral.
Habit
How well a gemstone resists scratches. Usually expressed in terms of the Mohs scale, with diamond the hardest (10) and talc the softest (1).
Hardness
A characteristic enclosed within a gem- stone, 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. Usually created by environmental changes during crystal growth.
Liquid inclusion
How well a gemstone resists breaking and chipping.
Toughness
Atoms in a gem that aren’t part of its essential chemical composition.
Trace elements
Change in a gem’s crystal direction during or after growth.
Twinning
Location of a change in crystal growth direction.
Twinning plane
A hollow cavity in a gem, usually filled with a liquid and a gas.
Two-phase inclusion
The smallest group of atoms with both the characteristic chemical composition and crystal structure of a mineral.
Unit cell
An aggregate made up of individual crystals visible under magnification.
Microcrystalline
A flat break in a gemstone caused by concentrated included minerals parallel to a twinning plane.
Parting
Ratio of the weight of a material to the weight of an equal volume of water.
Specific gravity (SG)
How well a gemstone resists light, heat, and chemicals.
Stability
Damage caused by sudden, extreme temperature changes.
Thermal shock
A hollow cavity in a gem, filled with a liquid, a gas, and one or more crystals.
Three-phase inclusion
Crystalline minerals
are classified into seven crystal systems, depending on the symmetry of their unit cells.
The exterior shape of a crystal
often indicates its symmetry at an atomic level.
Some gems are rare because they require
rare chemical elements or unique geological conditions, or both, to form.
Twinning can result from a change
during crystal growth, or from cooling or pressure after the crystal has grown.
Inclusions in a colored stone
can help identify it as natural or manmade,untreatedor treated.
Large inclusions can
dramatically affect beauty, value, and durability.
Many rough gemstones
have recognizable crystal shapes that can help identify them.
The shapes and sizes of rough gem crystals
affect the shapes and sizes of finished stones.
A gem’s density and specific gravity
depend on the types of atoms it’s made of, and how closely they’re packed together.
The different ways a gem breaks
are called cleavage, parting, and fracture.
A gem’s crystal structure determines
its reaction to external stress.
Most gems are
minerals
To be a gem, a mineral must be
beautiful, durable, and rare.
Many colored stones are mined
by independent miners using small-scale mining methods.
An appealing name can make a gem
much easier to sell.
Changes that take place in a gem’s source country
can drastically affect its availability and price.
Treatments improve
the marketability and availability of many gems.
In spite of treatment that’s sometimes extensive,
emerald is the number one colored stone by value imported into the US.
Most consumers accept gem treatments
if they’re ethically disclosed and explained in a positive way.
Lacking a regular crystal structure.
Amorphous
The basic structural unit of all matter.
Atom
Kinds and relative quantities of atoms that make up a material.
Chemical composition
A substance that consists of atoms of only one kind.
Chemical element
Regular, repeating internal arrangement of atoms in a material.
Crystal structure
A broad gem category based on chemical composition and crystal structure.
Gem species
A subcategory of species, based on color, transparency, or phenomenon.
Gem variety
A family of gems from several closely related mineral species.
Group
Composed of, or arising from, non-living matter.
Inorganic
A natural, inorganic substance with a char- acteristic chemical composition and usually characteristic structure.
Mineral
Gems produced by natural processes, without human help.
Natural gems
Produced by, or derived from, a living organism.
Organic
A natural material composed of masses of mineral crystals of one or more kinds.
Rock
A mineral that contains the elements silicon and oxygen.
Silicate
Rocks are the raw materials of
gemstone formation.
Most colored stones form
in the earth’s continental crust.
Geological conditions during mountain building
are favorable for gem formation.
In igneous rocks, large crystals indicate
slow cooling, while small crystals indicate quick cooling.
Some types of volcanic rock
can transport existing gems from deep in the crust to the earth’s surface.
Some of the largest and most perfect gem crystals
come from pegmatites.
Superheated, pressurized water can
transfer minerals in solution and redeposit them in veins to form gems.
Mining economics are affected
by demand, political climate, labor costs, environment, accessibility, type of deposit, and mining methods.
An understanding of gemstone formation
helps prospectors pick the right places to look for gems.
Thousands of tons of rock may be eroded to yield only a few ounces of gem material.
may be eroded to yield only a few ounces of gem material.
Secondary deposits are easier to work than primary ones and might contain greater concentrations of gems.
easier to work than primary ones and might contain greater concentrations of gems.
Only the toughest gems survive to become part of secondary deposits.
survive to become part of secondary deposits.
A deposit where gems are eroded from their source rock, then transported away from the source and further concentrated.
Alluvial deposit
Localized changes caused by an igneous intrusion that takes place where the magma meets the surrounding rock. New minerals may form due to temperature changes or introduction of fluids from the magma.
Contact metamorphism
The earth’s innermost layer.
Core
The surface and outermost layer of the earth.
Crust
A deposit where gems are eroded from the source rock but remain in place close to the source.
Eluvial deposit
The wearing away and transport of rock materials by natural forces.
Erosion
A spherical, often hollow, mineral-lined cavity in rock.
Geode
Minerals dense enough to become concentrated and separated from lighter ones by the action of surface water.
Heavy minerals
Hot, high-pressure solution that can dissolve, transport, and deposit minerals from one place to another.
Hydrothermal fluid
Rock formed by the crystallization of molten material.
Igneous rock
Large mass of igneous rock that crystallizes underground without reaching the surface.
Intrusion
General term for any molten rock.
Magma
A layer between the earth’s crust and its core.
Mantle
Circulation in the mantle that drives the movement of the earth’s plates.
Mantle convection
Rock altered by heat and pressure, or by heated fluids from magma.
Metamorphic rock
An igneous rock formed from cooling, once-molten granite that follows fractures in its surrounding rock. It may contain very large crystals, and its shape is often thin and contorted.
Pegmatite
Workable alluvial deposit of gem minerals with economic potential.
Placer
A section of the earth’s rigid outer crust.
Plate
Study of the formation, structure, and
movement of the plates of the earth’s crust.
Plate tectonics
Crystallization of minerals from a gas.
Pneumatolysis
Gems found in the rock that carried them to the surface.
Primary deposit
Changes in rock type and minerals over a wide area, caused by heat and pressure of large-scale geological events.
Regional metamorphism
