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Frank's Dictionary of Gemology Terms
a Glossary of Terms used in Gemology

A CRYSTAL STRUCTURE
Is a regular, repeating, three-dimensional arrangement of bonded atoms.

CRYSTAL SYMMETRY
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

POLYMORPHS
Are alternative structures that can be possessed by a single chemical substance.

ISOMORPHOUS or ISOSTRUCTURAL MATERIALS
Are those that possess alternative atoms within the same basic crystalline structure.


HARDNESS
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.

TOUGHNESS
Is the ability of a material to resist the development of fracture or cleavage through the body of the material.

CLEAVAGE
Is the ability possessed by some crystalline materials to break along certain well-defined crystal structure planes to leave more or less flat faces.

FRACTURE
Is the random, non-directional breakage which occurs in most gemstones as a result of stress.

STABILITY
Is the ability of a material to resist physical or chemical alterations due to light, heat or chemical attack.

DENSITY
Is the mass per volume of a substance. It is measured as the weight of a material compared with its volume (ie cc/g)

SPECIFIC GRAVITY
Is the ratio of the weight of a substance to the weight of an equal volume of water.

ARCHIMEDES PRINCIPLE
An object totally immersed in water experiences an upward force equal to the weight of the water it displaces.

(Therefore SG may be calculated as

SG = weight of object in air/weight in air - weight in water

TRANSPARENT MATERIALS
Are those through which an object can be seen clearly and distinctly

TRANSLUCENT MATERIALS
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 the material

OPAQUE MATERIALS
Are those that allow no light whatsoever to pass through them

REFRACTION
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.)

REFRACTIVE INDEX (RI)
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 MATERIAL
Exhibits the same optical properties in all directions through the material

AN OPTICALLY ANISOTROPIC MATERIAL
Exhibits different optical properties in different directions through the material

BIREFRINGENCE
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 (TIR)
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.

REFLECTION
Is the return by a surface of some of the light which falls on that surface.

LUSTRE
Is a surface reflection, the brightness and quality of which depends on the RI and the polish or surface condition of the material

INTERNAL REFLECTION EFFECTS
Are caused by the reflection of light from features below the surface of a gemstone.

IRIDESCENCE
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.

DISPERSION
Is the splitting of white light into the component colours of the visible light spectrum whereby each wavelength is refracted at a differing angle.

DISPERSION VALUE
Is the amount of dispersion in a gemstone, which is the numerical difference in RI of a ray when measured at two different wavelengths.


BODY COLOUR
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 EFFECT
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.

FLUORESCENCE
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.

PHOSPHORESCENCE
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. These are;
· Long wave ultra violet (LWUV) having a principal wavelength of 365nm
· Short wave ultra violet (SWUV) having a principle wavelength of 254nm

A MINERAL
Is a natural, inorganic crystalline material, which is formed by geological processes.


ARTIFICIAL GEM MATERIALS
Are those that have been manufactured and which are therefore not formed naturally.

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.

TREATMENT
Is the process of artificially altering and improving the appearance and properties of gem materials.