A CRYSTAL STRUCTURE
Is a regular, repeating, three-dimensional arrangement of bonded atoms.
Is a means of describing the repetition of the structural arrangements
of atoms and bonds in a crystalline material.
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
2,3,4 or 6 times during one complete revolution.
A PLANE OF SYMMETRY
Is an imaginary mirror plane dividing a crystal structure into two parts;
either part can be reflected through the mirror and still appear identical
to the other part.
A REFERENCE AXIS
or CRYSTALLOGRAPHIC AXIS
Is an imaginary line through a crystal structure which indicates a repeat
direction of the lattice points and also their relative repeat distance
along that direction.
AN AMORPHOUS MATERIAL
Is a solid composed largely of randomly oriented atoms and molecules,
and which therefore exhibits no crystalline form or effects
Are alternative structures that can be possessed by a single chemical
ISOMORPHOUS or ISOSTRUCTURAL
Are those that possess alternative atoms within the same basic crystalline
Is the ability of a material to resist abrasion when a pointed fragment
of another substance is drawn across it without sufficient pressure
to develop cleavage.
Is the ability of a material to resist the development of fracture or
cleavage through the body of the material.
Is the ability possessed by some crystalline materials to break along
certain well-defined crystal structure planes to leave more or less
Is the random, non-directional breakage which occurs in most gemstones
as a result of stress.
Is the ability of a material to resist physical or chemical alterations
due to light, heat or chemical attack.
Is the mass per volume of a substance. It is measured as the weight
of a material compared with its volume (ie cc/g)
Is the ratio of the weight of a substance to the weight of an equal
volume of water.
An object totally immersed in water experiences an upward force equal
to the weight of the water it displaces.
(Therefore SG may be calculated
SG = weight of object in
air/weight in air - weight in water
Are those through which an object can be seen clearly and distinctly
Are those that disrupt the passage of light so that an object can be
viewed indistinctly or not at all, but light can always be seen through
Are those that allow no light whatsoever to pass through them
Is the change in the direction in which light travels when it passes
from one medium into another of differing optical density (except where
incident at 90 degrees to the interface.)
Is a number which indicates the property of the refraction of light
in a material and which is derived from the laws of refraction.
AN OPTICALLY ISOTROPIC
Exhibits the same optical properties in all directions through the material
AN OPTICALLY ANISOTROPIC
Exhibits different optical properties in different directions through
Is the maximum numerical difference between the two refractive indices
of an anisotropic material.
DOUBLE REFRACTION (DR)
Is the optical doubling effect, which is caused by the splitting of
light into two separate components by an optically anisotropic material.
AN OPTIC AXIS
Is a direction through an anisotropic crystal structure parallel to
which single refraction occurs.
TOTAL INTERNAL REFLECTION
Occurs when incident light meets the interface between two media of
differing optical density at an angle greater than the critical angle,
the light remains within the optical denser material.
Is the return by a surface of some of the light which falls on that
Is a surface reflection, the brightness and quality of which depends
on the RI and the polish or surface condition of the material
Are caused by the reflection of light from features below the surface
of a gemstone.
Is a colour effect seen when light suffers interference by reflection
from thin films or by diffraction so that some light waves are removed
and others are reinforced.
Is the splitting of white light into the component colours of the visible
light spectrum whereby each wavelength is refracted at a differing angle.
Is the amount of dispersion in a gemstone, which is the numerical difference
in RI of a ray when measured at two different wavelengths.
In a gemstone is derived from;
· The light in which it is viewed
· The removal by the gemstone of some of the colour spectrum
from that light
Our perception of the residual light
THE COLOUR CHANGE
Occurs when, illuminating certain stones with two slightly differing
incident "white" light sources, different colour sensations
are produced in the brain.
PLANE POLARISED LIGHT
Is light whose rays all vibrate parallel to a single plane.
is the general term used to describe the emission of 'cold' non incandescent
light by a substance while it is being excited by radiation of shorter
wavelength and higher energy.
Is the term used to describe a material, which continues to fluoresce
(slowly diminishing) after the source of excitation has been removed.
ULTRA VIOLET LIGHT
Is that part of the electromagnetic spectrum which falls between 400nm
and 10nm. In gemmology testing two types of ultra violet light are used.
· Long wave ultra violet (LWUV) having a principal wavelength
· Short wave ultra violet (SWUV) having a principle wavelength
Is a natural, inorganic crystalline material, which is formed by geological
ARTIFICIAL GEM MATERIALS
Are those that have been manufactured and which are therefore not formed
SYNTHETIC GEM MATERIALS
Are those artificial materials that possess the same composition and
structure as a natural inorganic mineral counterpart.
IMMITATION (OR SIMULANTS)
Gem materials are natural or artificial materials that are used to imitate
the effect, colour and appearance of other gem materials without possessing
their chemical and physical properties.
Is the process of artificially altering and improving the appearance
and properties of gem materials.