GEM-A EXAM STUDY GUIDE
ABSORB, ABSORPTION - Answer -Visible light is absorbed when electrons in a
material interact with the light to convert it into invisible heat energy; this light is
therefore not transmitted all the way through the material.
ABSORPTION SPECTRUM - Answer -A pattern of dark bands or lines in the visible
light spectrum, seen when light from certain gem materials is viewed through a
spectroscope.
ADAMANTINE LUSTRE - Answer -A very bright and reflective lustre, typically
displayed by diamond.
ALLOCHROMATIC - Answer -Allochromatic materials owe their body colour to the
presence of certain chemical elements which do not form an essential part of the
chemical composition of the material (see also idiochromatic).
ALLUVIAL DEPOSIT - Answer -Sedimentary rock deposit formed by the action of water
eroding and transporting rock material. 'Placer' deposits or 'gem gravels' are mined from
certain alluvial deposits which are, or once were, river beds or lake beds.
AMORPHOUS - Answer -An amorphous material is composed largely of randomly-
oriented atoms, molecules or minute crystalline sectors. The material therefore exhibits
no overall crystal form or effects.
ANALYSER - Answer -The upper polarizing filter fitted to polariscopes and certain
types of microscope (see polarization and polarizer).
ÅNGSTRÖM - Answer -Unit of length measurement for wavelengths of certain ranges
of electromagnetic radiation, including visible light, now largely superseded by the
nanometre (nm). 10 Å = 1nm
ANISOTROPIC - Answer -An optically anisotropic material exhibits different optical
properties in different directions through the material. All crystal structures other than
those in the cubic crystal system are optically anisotropic.
ANOMALOUS EXTINCTION EFFECT - Answer -('anomalous birefringence') When
viewing certain gemstones between crossed polarizers, a pattern of light and dark
extinction zones, bands or cross shapes may be seen within some stones; these may
move across the stone on rotation. This effect is due to internal strain in the material
which causes a certain amount of anisotropy in an otherwise isotropic structure.
ARCHIMEDES' PRINCIPLE - Answer -An object immersed in a fluid experiences an
upward force equal to the weight of the fluid it displaces.
,ARTIFICIAL - Answer -Not natural or naturally-formed; 'man-made'. Includes gem
materials and gem colour varieties that are not natural; gem materials that have been
manufactured (compare with synthetic).
ASTERISM - Answer -A star-like reflection of intersecting bands of light which is seen
from the surface of certain gems when they cut in a particular crystallographic
orientation as cabochons. It is an internal reflection effect caused by reflections from
two or more sets of parallel fibrous or channel inclusions (see chatoyancy).
ATOM - Answer -An atom is the smallest unit of a chemical element that can retain the
properties of that element. An atom consists of energy in the form of an electron cloud
around a central nucleus
AXIAL CROSS - Answer -Three or four intersecting crystallographic (reference) axes
which represent the repeat directions and repeat distances in a single crystal structure.
AXIS (pl: axes) - Answer -An imaginary line indicating a direction (not a position)
through a material (see axis of symmetry, optic axis, crystallographic axis).
AXIS OF SYMMETRY - Answer -An axis of symmetry is an imaginary line which
indicates a direction through a crystal structure about which the structure can be rotated
to appear identical two, three, four or six times during one complete rotation (see also
crystal symmetry, crystal system).
BIAXIAL - Answer -An anisotropic crystal having two directions (optic axes) along
which there is single refraction. Crystalline materials belonging to the orthorhombic,
monoclinic and triclinic crystal systems are optically biaxial.
BIREFRINGENCE - Answer -Birefringence is the maximum numerical difference
between the two refractive indices of an optically anisotropic material (see also double
refraction).
BODY COLOUR - Answer -Body colour in a gemstone is derived from the visible light
in which it is viewed, the absorption by the gemstone of some of the colour spectrum
(i.e. certain wavelengths) from that light, and our perception of the residual light (see
selective absorption and colouring elements).
BRILLIANCE - Answer -The degree of brightness resulting from reflection of light by a
gemstone when viewed through the crown facets. It is made up of light reflected from
the pavilion facets and from the surface of the crown.
BRILLIANT (cut) - Answer -A faceted gemstone cut which produces a high degree of
brilliance; favoured for diamond to make good use of its dispersion.
BRUTING - Answer -A method of shaping one diamond with another. A sawn, cleaved
or whole diamond is mounted in a lathe and rotated. A second diamond is held firmly
, against the rotating diamond to bring it to the required shape for the next stage in
fashioning.
CABOCHON (cut) - Answer -A non-faceted gemstone cut which essentially consists of
one or two domed, polished surfaces.
CAMEO - Answer -Carving in relief, usually of shell or agate, which utilises the
differently coloured layers to good effect.
CARAT (ct) - Answer -A unit of weight for gem materials. One carat = 0.200 grams
CHATOYANCY - Answer -Chatoyancy, or cat's-eye effect, is an internal reflection
effect which appears as a bright band of light across the surface of certain gemstones
when cut as cabochons. The reflection is from fine inclusions, all parallel to one
direction, and is visible when the base of the cabochon is cut in parallel with the
direction of the inclusions.
CHEMICAL BOND - Answer -A chemical bond is a link between neighbouring atoms
caused by the interaction of electrons.
CHEMICAL COMPOUND - Answer -A chemical compound is a single substance
produced by the chemical bonding of two or more different chemical elements.
CHEMICAL ELEMENT - Answer -An element is a substance composed entirely of
chemically identical atoms. It cannot be broken down chemically.
CHEMICAL FORMULA - Answer -A chemical formula is a means of expressing the
composition of a substance in terms of the proportions of its component chemical
elements, using internationally recognised chemical symbols.
CHEMISTRY - Answer -Chemistry is the study of chemical elements and bonding, the
chemical changes that take place and chemical compounds that are formed.
CHROMATIC ABERRATION - Answer -Dispersion effect seen as colour fringes around
images when using some magnifying lenses of simple design. Doublet or triplet lenses
are manufactured in order to correct this.
CLEAVAGE - Answer -The ability, possessed by some crystalline gem materials, to
split along certain well-defined crystal structure planes to leave more or less flat
cleavage faces. Diamond may be cleaved during fashioning.
COLOUR - Answer -Colour is an effect in the brain produced by reaction to visible light
that reaches the back of the eye. The reaction triggers messages to the brain and
colour is 'seen'. Most colour is perceived after modification of white light (see differential
absorption, body colour, iridescence).
ABSORB, ABSORPTION - Answer -Visible light is absorbed when electrons in a
material interact with the light to convert it into invisible heat energy; this light is
therefore not transmitted all the way through the material.
ABSORPTION SPECTRUM - Answer -A pattern of dark bands or lines in the visible
light spectrum, seen when light from certain gem materials is viewed through a
spectroscope.
ADAMANTINE LUSTRE - Answer -A very bright and reflective lustre, typically
displayed by diamond.
ALLOCHROMATIC - Answer -Allochromatic materials owe their body colour to the
presence of certain chemical elements which do not form an essential part of the
chemical composition of the material (see also idiochromatic).
ALLUVIAL DEPOSIT - Answer -Sedimentary rock deposit formed by the action of water
eroding and transporting rock material. 'Placer' deposits or 'gem gravels' are mined from
certain alluvial deposits which are, or once were, river beds or lake beds.
AMORPHOUS - Answer -An amorphous material is composed largely of randomly-
oriented atoms, molecules or minute crystalline sectors. The material therefore exhibits
no overall crystal form or effects.
ANALYSER - Answer -The upper polarizing filter fitted to polariscopes and certain
types of microscope (see polarization and polarizer).
ÅNGSTRÖM - Answer -Unit of length measurement for wavelengths of certain ranges
of electromagnetic radiation, including visible light, now largely superseded by the
nanometre (nm). 10 Å = 1nm
ANISOTROPIC - Answer -An optically anisotropic material exhibits different optical
properties in different directions through the material. All crystal structures other than
those in the cubic crystal system are optically anisotropic.
ANOMALOUS EXTINCTION EFFECT - Answer -('anomalous birefringence') When
viewing certain gemstones between crossed polarizers, a pattern of light and dark
extinction zones, bands or cross shapes may be seen within some stones; these may
move across the stone on rotation. This effect is due to internal strain in the material
which causes a certain amount of anisotropy in an otherwise isotropic structure.
ARCHIMEDES' PRINCIPLE - Answer -An object immersed in a fluid experiences an
upward force equal to the weight of the fluid it displaces.
,ARTIFICIAL - Answer -Not natural or naturally-formed; 'man-made'. Includes gem
materials and gem colour varieties that are not natural; gem materials that have been
manufactured (compare with synthetic).
ASTERISM - Answer -A star-like reflection of intersecting bands of light which is seen
from the surface of certain gems when they cut in a particular crystallographic
orientation as cabochons. It is an internal reflection effect caused by reflections from
two or more sets of parallel fibrous or channel inclusions (see chatoyancy).
ATOM - Answer -An atom is the smallest unit of a chemical element that can retain the
properties of that element. An atom consists of energy in the form of an electron cloud
around a central nucleus
AXIAL CROSS - Answer -Three or four intersecting crystallographic (reference) axes
which represent the repeat directions and repeat distances in a single crystal structure.
AXIS (pl: axes) - Answer -An imaginary line indicating a direction (not a position)
through a material (see axis of symmetry, optic axis, crystallographic axis).
AXIS OF SYMMETRY - Answer -An axis of symmetry is an imaginary line which
indicates a direction through a crystal structure about which the structure can be rotated
to appear identical two, three, four or six times during one complete rotation (see also
crystal symmetry, crystal system).
BIAXIAL - Answer -An anisotropic crystal having two directions (optic axes) along
which there is single refraction. Crystalline materials belonging to the orthorhombic,
monoclinic and triclinic crystal systems are optically biaxial.
BIREFRINGENCE - Answer -Birefringence is the maximum numerical difference
between the two refractive indices of an optically anisotropic material (see also double
refraction).
BODY COLOUR - Answer -Body colour in a gemstone is derived from the visible light
in which it is viewed, the absorption by the gemstone of some of the colour spectrum
(i.e. certain wavelengths) from that light, and our perception of the residual light (see
selective absorption and colouring elements).
BRILLIANCE - Answer -The degree of brightness resulting from reflection of light by a
gemstone when viewed through the crown facets. It is made up of light reflected from
the pavilion facets and from the surface of the crown.
BRILLIANT (cut) - Answer -A faceted gemstone cut which produces a high degree of
brilliance; favoured for diamond to make good use of its dispersion.
BRUTING - Answer -A method of shaping one diamond with another. A sawn, cleaved
or whole diamond is mounted in a lathe and rotated. A second diamond is held firmly
, against the rotating diamond to bring it to the required shape for the next stage in
fashioning.
CABOCHON (cut) - Answer -A non-faceted gemstone cut which essentially consists of
one or two domed, polished surfaces.
CAMEO - Answer -Carving in relief, usually of shell or agate, which utilises the
differently coloured layers to good effect.
CARAT (ct) - Answer -A unit of weight for gem materials. One carat = 0.200 grams
CHATOYANCY - Answer -Chatoyancy, or cat's-eye effect, is an internal reflection
effect which appears as a bright band of light across the surface of certain gemstones
when cut as cabochons. The reflection is from fine inclusions, all parallel to one
direction, and is visible when the base of the cabochon is cut in parallel with the
direction of the inclusions.
CHEMICAL BOND - Answer -A chemical bond is a link between neighbouring atoms
caused by the interaction of electrons.
CHEMICAL COMPOUND - Answer -A chemical compound is a single substance
produced by the chemical bonding of two or more different chemical elements.
CHEMICAL ELEMENT - Answer -An element is a substance composed entirely of
chemically identical atoms. It cannot be broken down chemically.
CHEMICAL FORMULA - Answer -A chemical formula is a means of expressing the
composition of a substance in terms of the proportions of its component chemical
elements, using internationally recognised chemical symbols.
CHEMISTRY - Answer -Chemistry is the study of chemical elements and bonding, the
chemical changes that take place and chemical compounds that are formed.
CHROMATIC ABERRATION - Answer -Dispersion effect seen as colour fringes around
images when using some magnifying lenses of simple design. Doublet or triplet lenses
are manufactured in order to correct this.
CLEAVAGE - Answer -The ability, possessed by some crystalline gem materials, to
split along certain well-defined crystal structure planes to leave more or less flat
cleavage faces. Diamond may be cleaved during fashioning.
COLOUR - Answer -Colour is an effect in the brain produced by reaction to visible light
that reaches the back of the eye. The reaction triggers messages to the brain and
colour is 'seen'. Most colour is perceived after modification of white light (see differential
absorption, body colour, iridescence).
