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
A constant formation and recycling process that creates new rock from old.
Rock cycle
Gems found away from their primary source.
Secondary deposit
Rock produced from the eroded and weathered remains of existing rocks.
Sedimentary rock
A process that occurs when two of the earth’s plates collide, forcing one under the other.
Subduction
A mineral deposit that occupies an existing fissure or fracture in the rock.
Vein
Pertaining to igneous activity at the earth’s surface, where magma erupts through a volcano or a fissure.
Volcanic
A “foreign crystal” that formed in unrelated rocks and was brought to the surface as a passenger in magma.
Xenocryst
Color is the most important factor in determining a colored gemstone’s
desirability, marketability, and value.
Light is a form of energy that
travels in waves.
Most gemcolor is caused by selective absorption, which is influenced by
chemical composition and crystal structure.
Some colored gems can be identified by the way they
absorb visible light.
Because of differences in crystal structure, the same transition element can
cause different colors in different gems.
Chromium causes the most desirable
red and green gem colors.
Iron causes a greater variety of gem colors
than any other transition element.
Charge transfer is responsible for most
blue and yellow gem colors.
Treatment can alter or initiate some
color-causing charge transfer reactions.
Without exposure to natural or artificial radiation,
color centers can’t cause color in gems.
Only doubly refractive gems can be
pleochroic.
Interference produces either darkness
or the brightening of one or more colors.
Opal’s play-of-color is produced by the interaction of light with the gem’s microscopic internal structure of
stacked silica spheres.
Light reflecting from inclusions
causes several phenomenal effects in gems.
A pattern of dark vertical lines or bands shown by certain gems when viewed through a spectroscope.
Absorption spectrum
The cloudy bluish white light in a moonstone, caused by scattering of light.
Adularescence
A gem colored by trace elements in its crystal structure.
Allochromatic
Crossing of chatoyant bands, creating a star in the dome of a cabochon.
Asterism
A glittery effect caused by light reflecting from small, flat inclusions within a gemstone.
Aventurescence
A process where the electrons that selectively absorb light are passed back and forth between neighboring impurity ions.
Charge transfer
Bands of light in certain gems, caused by reflection of light from many parallel, needle-like inclusions or hollow tubes.
Chatoyancy
A small defect in the atomic structure of a material that can absorb light and give rise to a color.
Color center
A distinct change in gem color under different types of lighting.
Color change
A special kind of interference phenomenon that breaks up white light into its spectral hues.
Diffraction
When a gem’s crystal structure splits light into two rays that each travel at a slightly different speed and direction.
Double refraction
Emission of visible light by a material when it’s stimulated by ultraviolet radiation.
Fluorescence
A gem colored by an element that is an essential part of its chemical composition.
Idiochromatic
Interaction of two light rays traveling in the same path.
Interference
A process where two impurity atoms separated by another atom can still exchange electrons to selectively absorb light.
Intervalence charge transfer
A rainbow effect created when light is broken up into spectral hues by thin layers.
Iridescence
A broad flash of color in labradorite feldspar that disappears when the gem is moved.
Labradorescence
A two-toned effect seen when a chatoyant gem is positioned at right angles to a light source.
Milk and honey
Direction of single refraction in a doubly refractive gem.
Optic axis
Iridescence seen in some natural and cultured pearls and mother-of-pearl.
Orient
An unusual optical effect displayed by a gem.
Phenomenon
The flashing rainbow colors in opal.
Play-of-color
When a gem shows different body-
colors from different crystal directions.
Pleochroism
Change in speed and possible change in direction of light as it travels from one material to another.
Refraction
Process by which a material absorbs some components of visible light and transmits others.
Selective absorption
When a gem’s crystal structure doesn’t change incoming light other than by refraction and absorption.
Single refraction
Elements that can selectively absorb some wavelengths of visible light and produce color in gems.
Transition elements
Knowing whether a gem is natural or synthetic is
crucial when buying or selling colored stones.
Imitations can be made of a variety of materials
including natural or synthetic versions of other gems.
The process used to produce a synthetic gem can
determine its value.
Most synthetic gem crystals are produced by either
a melt or a solution process.
Solution processes are generally
slower and more expensive than melt processes.
Solution processes can synthesize gems that closely
resemble natural gems.
Internal and external characteristics can help trained observers
separate natural from synthetic gems.
Gemologists play an important role in ensuring that disclosure
becomes the industry standard.
Two or more separate pieces of material joined to form a unit.
Assembled stone
A cylindrical synthetic crystal produced by the flame-fusion process.
Boule
A process in which finely ground powder is heated, sometimes under pressure, to produce a fine-grained solid material.
Ceramic process
Clearly and accurately informing customers about the nature of the goods they buy.
Disclosure
Two separate pieces of material fused or cemented together to form a single assembled stone.
Doublet
A process in which powdered chemi- cals are dropped through a high-temperature flame onto a rotating pedestal to produce a synthetic crystal.
Flame fusion
A melt process where a heating unit passes over a rotating solid rod of chemicals until it forms a synthetic crystal.
Floating zone
A process in which nutrients dissolve in heated chemicals, then cool to form synthetic crystals.
Flux growth
A process in which nutrients dissolve in a water solution at high temperature and pressure, then cool to form synthetic crystals.
Hydrothermal growth
Any material that looks like a natural gem and is used in its place.
Imitation gem
A synthetic-crystal growth method in which the chemical mixture is melted, then recrystallized.
Melt process
A process in which the synthetic crystal grows from a seed that is dipped into a chemical melt, then pulled away as it gathers material.
Pulling
A tiny crystal used as a template to control the size, speed, or direction of growth and the shape of a growing synthetic crystal.
Seed crystal
A synthetic-crystal growth method that uses cooling pipes around an interior of melted chemical ingredients.
Skull melt
A growth method in which the synthetic crystal grows from a dissolved chemical mixture, sometimes at high temperature and pressure.
Solution process
A synthetic-crystal growth method that starts without a seed crystal.
Spontaneous nucleation
A laboratory creation with essentially the same chemical composition, crystal structure, and properties as its natural counterpart.
Synthetic gem
A single assembled stone made from three separate pieces of material fused or cemented together, or from two pieces and a colored cement layer.
Triplet
Treatments that improve gem appearance and quality increase the supply of
commercially important gems.
Treatments that make gems more available and affordable are an important part of
today’s gem and jewelry market.
All treatments—even suspected or probable treatments
should be disclosed as part of every transaction.
Gems that are commonly heat treated include corundum, tanzanite, zircon, topaz, aquamarine, and amber.
aquamarine, and amber.
Heat treatment can alter color and create or eliminate
phenomena and inclusions.
Lattice diffusion with titanium produces a shallow layer of color
that can be removed with minor re polishing.
The color resulting from lattice diffusion with beryllium might
penetrate the entire stone.
Almost all emeralds on the market today are
fracture filled.
Irradiated color is not always stable, and exposure to heat or light can
destroy it.
Commonly irradiated gems include topaz, tourmaline, and
cultured pearl.
Bleaching is a fairly common
pearl treatment.
Jadeite is often treated with a combination of bleaching and
impregnation.
Dyeing is one of the
oldest gem treatments.
Commonly dyed gems include cultured pearl, lapis lazuli,
chalcedony, and coral.
A treatment that uses chemicals to lighten or remove color.
Bleaching
Filling of pores or other openings with melted wax, resin, polymer, or plastic to improve appearance and stability.
Colorless impregnation
A treatment that adds color or affects color by deepening it, making it more even, or changing it.
Dyeing
Using a filler to conceal fractures and improve the apparent clarity of a gem.
Fracture (fissure) filling
A chemical that treaters mix with some resins to cause them to solidify.
Hardener
Exposing a gem to rising tempera- tures for the purpose of changing its appearance.
Heat treatment
Exposing a gem to radiation to change or improve its color.
Irradiation
A treatment in which a gem is exposed to high temperatures and chemicals to allow penetration of color-causing elements.
Lattice diffusion
An oxygen-rich environment that surrounds a gem during heat treatment, causing it to absorb oxygen.
Oxidizing environment
A liquid filling material that’s very durable when it dries.
Polymer
A rapid heating and cooling process that produces fractures in a stone so it will accept dye.
Quench crackling
An oxygen-poor environment that surrounds a gem during heat treatment, causing it to lose oxygen.
Reducing environment
A clear, viscous substance that’s used to fill fractures in gemstones.
Resin
Heating a wrapped opal until smoke or ash penetrates its surface to darken it and bring out its play-of-color.
Smoke treatment
Soaking an opal in a hot sugar solution and then in sulfuric acid to darken it and bring out its play-of-color.
Sugar treatment
Altering a gem’s appearance by applying backings, coatings, or coloring agents like paint.
Surface modification
Any human-controlled process, beyond cutting and polishing, that improves the appearance, durability, or value of a gem.
Treatment
Different buyers require different goods to suit their customers’ needs.
customers’ needs.
The highest demand is for
mass-market-quality gem rough.
Many gems leave their source countries through
unofficial channels.
Stones pass through many hands before they reach the consumer, and there’s a
price increase at every step.
Many conditions threaten the stability of the
colored stone market.
Commercial-quality stones are generally
calibrated, and judged more by the price per stone than by their individual quality factors.
Cutting standards are generally looser for
low-end goods.
Color, cut, and clarity are important factors for
middle-market gems.
A parcel typically contains a range of qualities, and the best stones often sell for a
lot more than the remainder of the parcel.
Because retailers deal directly with consumers, they must have
complete confidence in their wholesalers.
Main attributes of a fine stone
rarity, exceptional color, and large size.
The documented origin of a fine-quality gem can be a powerful sales tool and have an impact
on its value.
Gemstone sizes cut to fit standard mountings.
Calibrated sizes
A document that indicates a stone’s geographic origin, based on its inclusions and trace element chemistry.
Certificate of origin
Market sector where average- quality gemstones are used in mass-market jewelry.
Commercial market
A unique piece designed and created for a particular customer, often around specially chosen stones.
Custom-made jewelry
A gem dealer’s term for a random sample from a parcel of gemstones, often used to assess the parcel’s overall quality.
Cut
A manufacturer who produces faceted stones, cabochons, or carvings.
Cutter
A city, region, or country with a large number of gemstone manufacturers.
Cutting center
Gemstone rough that’s transparent enough and of high enough quality to produce faceted gems.
Facet grade
Non-standard cutting, usually applied to large, important stones for use in expensive jewelry where standard size is not a consideration.
Free-size
A specific rough gemstone quality range, usually determined by color, size, clarity, and price.
Grade
Market sector where fine - quality, expensive gemstones are used in unique, handcrafted jewelry pieces.
High-end market
In mining terms, theft of a mine’s production by its workers.
High grading
A discounted price for buying an entire parcel or a substantial part of it.
Lot price
Market sector where better-quality gemstones are used in well-finished, moderately priced jewelry pieces.
Middlemarket
A mixture of gem qualities that represents unsorted production from a particular mine.
Mine lot (mine run)
The geographical place where a gem was mined.
Origin
A quantity of stones, sometimes of similar size and quality, perhaps from a single mine, but often from various sources, that’s offered for sale together.
Parcel
A premium price for selecting stones from a parcel.
Pick price
A gem-producing area, or a particular mine in that area.
Source
A company or individual that supplies gems to jewelry manufacturers and retailers.
Wholesaler
