Gem-A

ANOMALOUS EXTINCTION EFFECT

('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.

CONCHOIDAL FRACTURE

A broken surface with concentric, raised surface markings looking similar to the ridges on a seashell (see also fracture).

CHEMICAL BOND

A chemical bond is a link between neighbouring atoms caused by the interaction of electrons.

CHEMICAL COMPOUND

A chemical compound is a single substance produced by the chemical bonding of two or more different chemical elements.

CHEMICAL FORMULA

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.

DOUBLET

A composite gemstone consisting of two parts of the same or different materials joined together, e.g. garnet-topped doublet. Also a loupe with two component lenses.

COVALENT BONDING

A covalent bond is an electronic link resulting from the sharing of electrons between atoms.

CRYSTAL FORM

A crystal form is a set of crystal faces related by the symmetry of the crystal

CRYSTAL LATTICE

A crystal lattice is an imaginary, regularly-repeating array of points or dots representing the repeat pattern of atoms, molecules and bonding in a real crystal structure.

CRYSTAL STRUCTURE

A crystal structure is the regular, repeating, three-dimensional arrangement of bonded atoms.

CRYSTALLOGRAPHIC AXES

A crystallographic or reference axis is an imaginary line that represents a particular direction of repeat of atoms and bonds through a crystal structure and also the relative repeat distance along that direction.

DIFFRACTION GRATING SPECTROSCOPE

A diffraction 'grating' consisting of very fine, closely-spaced parallel lines printed or engraved on a glass, plastic or metal plate is used to produce an observable spectrum in this type of spectroscope.

BRILLIANT (cut)

A faceted gemstone cut which produces a high degree of brilliance; favoured for diamond to make good use of its dispersion.

FACET

A flat face cut and polished on a gemstone

DOPSTICK (or dop)

A holder in which a gemstone is set for cutting and polishing.

BRUTING

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.

HYDROTHERMAL DEPOSIT

A mineral occurrence formed by precipitation from hot watery solution; typically filling cracks and cavities in rocks.

CABOCHON (cut)

A non-faceted gemstone cut which essentially consists of one or two domed, polished surfaces.

ABSORPTION SPECTRUM

A pattern of dark bands or lines in the visible light spectrum, seen when light from certain gem materials is viewed through a spectroscope.

DIAMOND PIPE

A pipe-shaped occurrence within the Earth's crust of igneous rock containing diamond.

CRYSTAL FACE

A plane surface of a crystal structure where unrestricted crystal growth has ceased. Crystal faces tend to be parallel to those crystal structure layers that are most densely packed with atoms.

FIRE

A popular term for the spectral colours displayed by a gemstone with a significant degree of dispersion; seen well in some faceted stones.

ASTERISM

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).

CRYPTOCRYSTALLINE

A substance consisting of masses of minute crystals, too small to be seen under an ordinary optical microscope (see microcrystalline, polycrystalline).

CARAT (ct)

A unit of weight for gem materials. One carat = 0.200 grams

ADAMANTINE LUSTRE

A very bright and reflective lustre, typically displayed by diamond.

CRYSTAL

A word used to describe a single crystalline material with plane crystal faces; any material with single, continuous internal crystal structure including artificial single-crystal materials. The term refers to fashioned and rough materials.

ALLOCHROMATIC

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).

CRYSTAL SYMMETRY

An aid to description and classification which describes the repetition of the structural arrangement of atoms and bonds in a crystalline material (see axis of symmetry, plane of symmetry, crystal structure, crystal system).

AMORPHOUS

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.

BIAXIAL

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.

ATOM

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

AXIS OF SYMMETRY

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).

CHEMICAL ELEMENT

An element is a substance composed entirely of chemically identical atoms. It cannot be broken down chemically.

AXIS (pl: axes)

An imaginary line indicating a direction (not a position) through a material (see axis of symmetry, optic axis, crystallographic axis).

COLOUR

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).

COMPOSITE

Composite gem materials are those assembled artificially from two or more natural, synthetic or artificial components, in such a way as to give the impression of a single item (see doublet and triplet).

CRYSTAL HABIT

Crystal habit is the overall, natural external shape taken by a crystal. It is the development of a crystal form, or the relative development of two or more different crystal forms.

DIRECTIONAL PROPERTIES

Crystalline materials exhibit directional properties. These properties are controlled by the crystal structure of the material. Examples of directional properties include hardness, pleochroism and surface markings on crystals.

DENSITY

Density is measured as the weight of a material compared with its volume. It is defined as the mass per unit volume of a substance (see relative density and specific gravity).

CRYSTALLINE

Describes those materials that possess crystal structure.

DICHROISM

Dichroism is pleochroism in uniaxial crystal structures, through which the two principal vibration directions show a differing colour or shade of colour (see dichroscope, trichroism).

CHROMATIC ABERRATION

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.

DISPERSION

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. The measured amount of dispersion in a gemstone is the difference in refractive index of light measured at two different selected wavelengths (see 'fire').

GAMMA RAYS

Electromagnetic radiations having wavelengths shorter than those of X-rays but longer than those of cosmic rays. Gamma rays may be used to alter the colour of certain gem materials.

ELECTRONS

Electrons are very small particles which are a component of atoms. Each electron carries a single negative electrical charge. They are involved in chemical bonding and interact with certain wavelengths of electromagnetic radiation, including visible light waves (see atom).

FRACTURE

Fracture Is the random, non-directional breakage which occurs in most gemstones as a result of stress. Stress can be due to internal or external forces such as a sudden temperature change or a sharp knock (see also cleavage).

GRAIN(3)

Growth layers and visible structural planes within a crystal; especially applicable to diamond.

DIFFERENTIAL HARDNESS

Hardness which differs with the direction of scratching; due to the structural arrangement of planes and directions of atomic bonding in certain crystal structures.

IDIOCHROMATIC

Idiochromatic materials are those which owe their body colour to elements that are an essential part of the chemical composition of the material (see also allochromatic)

IMITATION

Imitation gem materials (simulants) are natural or artificial materials used to imitate the effects, colour and appearance of other gem materials without possessing their chemical nature and physical properties.

DURABILITY

In gem materials, a combination of hardness, toughness and stability

ARTIFICIAL

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).

DARK FIELD (illumination)

Observation (especially under a microscope) of transparent material against a dark background, with illumination from the sides rather than from behind or onto the top of the material.

FLUX-MELT TECHNIQUE

Production of synthetic gem-quality crystals whereby hot, molten fluid (a 'flux') is used to dissolve gem-producing materials which, on cooling, recrystallize to form the desired gem material.

FLAME FUSION PROCESS

Production of synthetic gem-quality crystals whereby material is melted (e.g. powder in a very hot flame) to form a single mass of crystalline solid, e.g. Verneuil synthesis of corundum.

HYDROTHERMAL TECHNIQUE

Production of synthetic gem-quality crystals whereby the ingredients are placed in water in a sealed container and heated. The resultant high pressure causes the liquid to dissolve the ingredients which then recrystallize onto 'seed plates' in cooler parts of the vessel.

ALLUVIAL DEPOSIT

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.

CRYSTAL SYSTEM

Simple classification of crystal structures based on essential crystal symmetry and crystallographic axes of reference.

FIRE MARKS

Small cracks at edges of facets, due to overheating caused by rapid polishing. Seen especially in synthetic Verneuil corundum.

CROWN

That part of a faceted gemstone above the girdle; the 'front' or 'top' of the gemstone (see also pavilion and facet).

EXTRAORDINARY RAY

The 'ray' of light waves that all vibrate parallel to any plane containing the optic axis is termed the extraordinary ray or E-ray (see also ordinary ray).

HARDNESS

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 or fracture (see durability and toughness).

CLEAVAGE

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.

ENERGY LEVELS

The behaviour and movement of electrons around the nucleus in an atom can be shown as concentric circles at different distances from the nucleus. These are called electron energy levels or 'shells'.

COLOUR CHANGE EFFECT

The colour change effect occurs when, upon illuminating certain stones with two slightly differing incident 'white' light sources, different colour sensations are produced in the brain.

BRILLIANCE

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.

EXTINCTION (optical)

The lack of visible light transmission through a transparent anisotropic material at certain positions when observed between crossed polarizers (see crossed polarizers, polariscope and anomalous extinction effect).

FLUORESCENCE

The most important kind of luminescence used by gemmologists. Fluorescence is the general term used to describe the emission of visible light by a substance whilst it is being illuminated by radiation of shorter wavelengths (higher energy). Illumination is usually by visible light, ultraviolet light, or X-rays (see also phosphorescence).

DOUBLE REFRACTION (DR)

The optical doubling effect caused by the splitting of visible light into two separate components by an optically anisotropic material (see also birefringence).

GIRDLE

The perimeter of a faceted gemstone dividing the crown from the pavilion

FOILING

The practice of fixing shiny metal foil onto the back of the stone or between the stone and a closed-back setting. The foil may be coloured (see treatment).

ELECTROMAGNETIC SPECTRUM

The range of electromagnetic radiation wavelengths from the longest, low frequency radio waves, through diminishing wavelengths to infrared, visible light, ultraviolet, x-rays and gamma-rays to the shortest of all, very high-frequency cosmic rays (the highest energy waves). In gemmology, the word spectrum is often used to denote merely the visible light spectrum and the absorption spectrum.

FREQUENCY

The rate at which waves or 'cycles' of energy, pass a static point; quoted in hertz, cycles/second (see wavelengths and electromagnetic spectrum)

HACKLY

The uneven and often jagged surface that occurs when a material such as nephrite jade is fractured. This texture typically occurs in fractures through tough materials composed of intergrown fibrous crystals.

ANALYSER

The upper polarizing filter fitted to polariscopes and certain types of microscope (see polarization and polarizer).

DIFFRACTION

This is a 'spreading' effect upon light that occurs when light waves pass an edge, pass through thin slits or very small holes. It can cause interference in white light to produce iridescence.

AXIAL CROSS

Three or four intersecting crystallographic (reference) axes which represent the repeat directions and repeat distances in a single crystal structure.

FASHIONED, FASHIONING

To fashion a gem material is to shape or cut (and usually polish), the material for adornment or ornamental use. Includes faceting and carving.

ELECTROMAGNETIC RADIATION

Transmitted electromagnetic energy, of any part of the electromagnetic spectrum, spoken of as waves or rays.

COLOUR FILTER

Transparent material with a particular selective absorption which is used to view the body colours of gem materials. The overall selective absorption through both the filter and the gem material may differ for different gem materials with the same body colour, allowing the gem materials to be distinguished.

CROSSED POLARIZERS

Two polarizing filters are 'crossed' when arranged one in front of the other and rotated until a minimum of light is transmitted to the eye (see extinction, polarizer and polariscope).

ÅNGSTRÖM

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

GRAIN(2)

Unit of pearl weight: one pearl grain equals 0.25 carat;

ABSORB, ABSORPTION

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.

IGNEOUS

When molten rock or magma cools and solidifies, the products are known as igneous rocks.

GRAIN(1)

An individual crystal or fragment making up a rock, polycrystalline material or artificial reconstructed material;

DICHROSCOPE

An instrument for detecting pleochroism in a material. Using either a calcite crystal or polarizing filters, it is constructed so that two pleochroic colours, or shades of colour, may be detected and compared side by side in a single view.

ARCHIMEDES' PRINCIPLE

An object immersed in a fluid experiences an upward force equal to the weight of the fluid it displaces.

ANISOTROPIC

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.

CONCHIOLIN (also called conchin)

An organic substance secreted by some invertebrate animal life including coral polyps and pearl-producing molluscs. Conchiolin is a constituent of pearl and shell.

ENHANCEMENT

Artificial alteration of the appearance and properties of gem material in order to improve its appearance, stability or saleability (see treatment).

CRITICAL ANGLE

At the interface between two media of different optical density, when the angle of incidence of a ray of light from within the optically denser medium is increased to such an extent that a refracted ray of light passes along the interface, then this angle of incidence is called the critical angle (see refractive index and incident).

BIREFRINGENCE

Birefringence is the maximum numerical difference between the two refractive indices of an optically anisotropic material (see also double refraction).

BODY COLOUR

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).

CAMEO

Carving in relief, usually of shell or agate, which utilises the differently coloured layers to good effect.

GROUP (mineral; gem)

Certain isomorphous 'families' of gem minerals which have variable composition; for example the garnet group. Mineral groups are subdivided into mineral species (see variety).

CHATOYANCY

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.

COLOURING ELEMENTS

Chemical elements within a material that are capable of causing selective absorption of visible light (see body colour).

CHEMISTRY

Chemistry is the study of chemical elements and bonding, the chemical changes that take place and chemical compounds that are formed.