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absorption spectra A distinctive pattern of dark absorption lines and broad absorption bands observed when light reflected from, or transmitted through, a gemstone is viewed through a spectroscope.

acid igneous rocks Igneous rocks with a high silica content and usually light in colour.

adamantine The highest classification of surface lustre or reflectivity which may be shown by a gemstone e.g. in faceted diamond.

adularescence (schiller) A white/blue shimmer which passes across the curved surface of a polished moonstone cabochon due to scattering of light from alternating parallel layers of potassium and sodium rich feldspars.

allochromatic Colouring due to a trace impurity which is not part of the basic chemical composition of the gem mineral or colouring resulting from coloured inclusions.

alluvial (deposits) Mineral deposits transported by streams from their original source and deposited at sites of lower fluid velocity.

amorphous Without crystalline structure or definite shape e.g. glass, amber, precious opal.

anhedral A term used to describe crystals which do not have well formed faces.

anomalous double refraction Irregular light and dark patches observed when singly refracting stones having internal strain are rotated under the polariscope e.g. diamond, garnet and some pastes. Synthetic spinel shows stripes called "tabby extinction".

Archimedes' Principle "When a body is immersed in a fluid, there is an apparent loss of weight equal to the weight of the fluid displaced".

asterism Formation of a "star" by reflection or scattering of light from sets of parallel fibrous inclusions which are aligned with specific crystal directions. Epiasterism refers to stars observed in reflected light; diasterism refers to stars observed in transmitted light.

atom The smallest subdivision of matter that retains the characteristics of the element.

aventurescence Specular reflections or spangles of light reflected from plate-like inclusions as a stone is rotated. Examples are reflections from fuschite mica in aventurine quartz, copper inclusions in labradorite sunstone or hematite inclusions in oligoclase sunstone.

Band Theory A theory explaining the cause of colour in materials whose electrons are entirely delocalised. If the band gap between atoms in the valency and conduction energy bands is greater than the energy range of visible light, an intrinsically colourless stone results e.g. topaz.

baroque Gem materials having an irregular shape e.g. baroque pearls.

basic igneous rocks Igneous rocks low in silica content and usually of dark colour.

bearding A series of fine fractures produced around the girdle of a diamond which has been bruted or polished too severely.

Becke line An immersion technique for estimating Refractive Index. When the focus of the microscope is raised, the bright edge moves into the medium having the highest Refractive Index. If the facet edges of a cut stone change from light to dark as the focus moves down from the liquid into the stone, then the RJ of the gemstone is greater than the RI of the liquid.

birefringence The numerical difference between the maximum and minimum Refractive Index in doubly refracting substances.

Bravais space lattice The basic building blocks or arrangement of atoms, ions or molecules from which all known crystal structures can be built. The 14 space lattices can be grouped into seven crystal systems according to their symmetry, the relative lengths of their edges and the angles between them.

Brewster angle When monochromatic light meets the flat surface of an optically denser medium, only those rays polarised in the plane of that surface will be reflected when the angle of incidence is normal to the associated refracted ray in that medium. This angle of incidence, called the Brewster angle of polarisation, may be measured to determine the Refractive Index of gemstones above the usual range of the refractometer.

brilliance The amount of light reflected from the surface and by internal reflection from a faceted gemstone.

cabochon A cut stone having a curved or domed top surface and a hollow, flat or convex base.

cameo A carving having the design in relief or raised above the background surface. Cameos are usually made in layered materials such as agate or shell so as to achieve contrasting colours.

carat A unit of weight for gemstones. There are five metric carats to the gram.

centre of symmetry A crystal is said to have a centre of symmetry when an imaginary line can be passed from any point on its surface through its centre, and a similar point on the surface is found at an equal distance beyond the centre.

chatoyancy (cat's eye) The narrow band of reflected light caused by reflection or scattering of incident light from parallel fibres, tubes or needles when a fibrous stone is cut as a cabochon. The bright band is at right angles to the direction of the fibres; as the stone is turned, the line moves across the stone in the same direction.

Chelsea Colour Filter A sheet of green, transparent optical glass which transmits only narrow wavelengths of light in the red and yellow/green, other wavelengths being absorbed. It is generally used to distinguish blue or green stones that contain some red wavelengths.

cleavage The tendency of a mineral crystal to split along certain planes which are parallel to actual or possible crystal faces, yielding more or less smooth, flat surfaces.

cohesion The electro-magnetic forces of attraction existing between the atoms or ions of a substance, as a result of which there is resistance to separation.

colour The response of the red, green and blue pigments in the cones of the eye to wavelengths received in the visible range.

colour centre A defect that causes light absorption, particularly a defect caused by radiation.

colour change The change in colour observed in some stones when they are viewed separately in daylight and incandescent light, e.g. chrysoberyl var. alexandrite.

colour zoning Variations in colour due to varying chemical composition during crystal growth. Colour zoning may be straight, following crystal forms (as in natural sapphire) or curved (as in flame fusion synthetic ruby). In parti-coloured stones, complete portions of the stone differ in colour.
composite stones Assembled stones consisting of two (doublet) or three (triplet) parts e.g. garnet-top doublet, natural/synthetic sapphire doublet, soude emerald triplet, opal triplet.

compound A chemical combination of two or more elements. The properties of a compound may be different to those of the combining elements.

contact twin A twin crystal in which a common plane is shared e.g. simple contact, rotational contact, repeated (lamellar or cyclic) twins.

contact liquid The liquid used to achieve optical contact between a gemstone and the refractometer window. The liquid used with glass refractometers is usually methylene iodide saturated with sulphur and tetraiodoethylene; it has a Refractive Index of 1.81.

covalent bonding Bonding between atoms, achieved by sharing of outer shell electrons so as to obtain a stable outer shell configuration in each atom. The covalent bond is the strongest of the chemical bonds.

critical angle The angle of incidence of a ray in the denser medium at which the ray is refracted 90 degrees in the less dense media. At incident angles greater than the critical angle, the ray is totally internally reflected.

crossed filters Illumination of a gemstone though a blue filter and observation through a red filter to detect red fluorescence caused by the blue light e.g. ruby, red spinel.

crown The portion of a faceted stone above the girdle.

cryptocrystalline Having a crystal size that is too small to observe with a normal optical microscope.

crystal A body bounded by flat surfaces on a definite plan which is an expression of the orderly internal atomic structure.

crystalline Possessing the regular structure and directional properties but not necessarily the regular geometrical shape of the crystal.

crystallographic axes Imaginary reference lines of infinite length, intersecting at the crystallographic origin, which are used to describe the angular relationships of the crystal faces. Axes parallel to the edges of the Unit Cell are normally used.

culet A small facet cut at the pointed junction of the pavilion facets to prevent damage at the point.

Czochralski pulling A technique for producing synthetic gemstones in which a seed crystal is touched to the surface of a melt and slowly withdrawn with rotation and precise temperature control to produce a single crystal.

dark-field illumination Light directed sideways into the gemstone, allowing inclusions to stand out brightly in relief from a dark background, without glare.

diaphaneity (transparency) The freedom with which light is transmitted through a substance. Categories used are transparent, translucent and opaque.

differential selective absorption Selective absorption of polarised rays in doubly refracting stones, according to direction in the stone, resulting in the stone appearing different in colour when viewed in different directions.

diffraction The deflection and breaking up of a beam of light at the edge of an opaque body or through a narrow aperture; white light is broken up into a band of pure spectral colours. In precious opal, diffraction of light occurs through the regularly stacked three-dimensional array of silica spheres of uniform size.

diffraction grating spectroscope A spectroscope in which the spectra are produced by diffraction and interference of the incident light as it passes through or is reflected from a finely ruled grating.

diffusion treatment A treatment in which a slurry of finely ground titanium and iron oxides is painted on the surface of nearly colourless sapphire and heated at high temperature for several days, allowing the iron and titanium to diffuse into the stone to produce a thin blue skin.

dispersion (fire) The power of a transparent substance to split white light into its spectral colours. Dispersion is measured numerically by the difference in refractive index between the B Fraunhofer line in the red and the G Fraunhofer line in the blue of the solar spectrum.

double refraction The splitting of a light ray entering a crystal (belonging to other than the cubic system) into two rays which are refracted or bent to a different extent as a result of their different vibration directions and velocities.

doubling The double image of back facets and inclusions observed when stones of high birefringence are examined with a hand lens e.g. in zircon and peridot.

elements Substances that cannot be split into simpler substances by chemical means. electromagnetic spectrum The range of radiated electromagnetic energy extending from long wavelength radio waves to short wavelength cosmic rays. The visible spectrum covers the range of wavelengths from 700nm (red) to 400nm (violet).

emission spectrum A pattern of bright spectral lines resulting from emission of energy when excited orbital electrons return to their ground state.

enantiomorph A crystal which has mirror image habits and optical characteristics e.g. right and left-handed quartz.

epigenetic (inclusions) Inclusions which develop after the host mineral has crystallised e.g. rutile inclusions in star sapphire and garnet, zircon "haloes" in almandine garnet.

euhedral A term used to describe crystals having a well-formed characteristic shape.

faceting The cutting of gemstones with a series of symmetrically placed flat faces. The upper portion of the faceted stone is called the crown and the lower portion is called the pavilion.

feldspars Rock-forming minerals consisting of complex silicates of aluminium and another element, usually potassium, sodium, calcium or barium. The most important gem feldspars are the alkaline feldspars and the plagioclase feldspars.

fire marks Small transverse cracks at facet edges caused by overheating resulting from rapid polishing e.g. in synthetic corundum.

fluorescence Emission of visible light when a mineral is exposed to radiations of shorter wavelength, such as ultraviolet light, X-rays or visible light of shorter wavelength than the emitted light. An activating element or lattice defect is necessary to produce the effect.

flux growth A method of producing synthetic gemstones in which a flux solvent, at high temperature and frequently at high pressure, is saturated with the required gemstone ingredients. Slow cooling results in crystal nucleation and growth. An example is lead fluoride/cryolite flux used in manufacture of synthetic corundum.

form (crystallographic form) All those faces which are identically related to the crystal axes e.g. bipyramid, prism, pinacoid, dome. A closed form encloses space; an open form does not itself enclose space.

fracture The surface obtained by breaking a substance in directions other than a cleavage or parting plane e.g. conchoidal, even, uneven, hackly.

gemmology The science of those minerals, and other rare materials, beautiful and durable enough for use as personal adornment and the embellishment of our personal possessions.

gemstone A material possessing beauty, durability and rarity.

glass An amorphous substance, usually resulting from rapid cooling of molten silicates. It may be natural (e.g. obsidian, moldavite) or man-made. Its properties do not vary with direction.

habit The characteristic shape of a mineral crystal or crystal aggregate. Habit can be described by the predominant crystal form e.g. prismatic, octahedral.

haloes Disc shaped fractures, sometimes showing two or four lobes, due to strain generated at crystal inclusions e.g. zircon haloes in sapphire.

hand lens (loupe) A lens or pair of lenses held in a short metal cylinder, giving a magnification of about 10 diameters and having some correction for chromatic and spherical aberration. It is used to examine the external and internal features of gemstones.

hardness The resistance to abrasion of a mineral when a pointed fragment of another substance is drawn across it without sufficient pressure to develop cleavage.

healed fractures Fractures occurring either during or after crystal growth which have filled with liquid and later partially healed, leaving liquid remnants or negative crystals. Healed fractures take characteristic forms in various gemstones e.g. rippled fractures in amethyst, fingerprints or feathers in sapphire and ruby.

heavy liquid A liquid used to determine Specific Gravity of stones. A stone will float in a heavy liquid of greater density than itself and sink in one of lower density. Liquids used include bromoform (SG 2.88) and methylene iodide (SG 3.32).

hemihedral A term applying to crystals which exhibit only half the number effaces required by their crystal system.

hemimorphism The property possessed by certain crystals of presenting different forms at opposite ends of a symmetry axis (generally the vertical crystal axis) e.g. different pyramids at each end of a tourmaline crystal.

holohedral A term describing a crystal which exhibits the full symmetry of its crystal system.

hot point tester An instrument which uses a heated wire element to determine the melting characteristics and odour of emitted vapours in gem materials. Usually used to determine characteristics of organic gem materials and their imitants e.g. amber and its imitants such as casein and bakelite plastics.

hydrothermal growth A method of producing synthetic gemstones where mineralised water, at high temperature and high pressure, is saturated with the required gemstone constituents and then slowly cooled, forming crystals.

hydrostatic SG method A method of determining the Specific Gravity (SG) of a gemstone by weighing it first in air and then totally immersed in a liquid. SG = (Weight in Air / Loss of Weight in Liquid) x SG of liquid. The liquid used is normally water of SG = 1.00 .

idiochromatic "Self colouring" caused by the major chemical constituents of the mineral e.g. iron in peridot, copper in malachite.

igneous rocks Rocks formed by the solidification of molten rock (magma) within or on the surface of the earth. Igneous rocks are classified as plutonic, hypabyssal or volcanic. Gem minerals found in igneous rocks include corundum (sapphire), diamond and peridot.

imitation (simulant) Stones, either natural or synthetic, which imitate in appearance a more valuable natural stone.

incident illumination Illumination in which the light rays are incident to and reflected from the surface of the gemstone being examined. Used for translucent and opaque stones and to examine surface features.

inclusions Internal features in gemstones. Some inclusions are characteristic of specific gemstones e.g. chrysotile horsetail in demantoid, curved striae in Verneuil synthetics.

inert Lack of response to stimulating radiation (e.g. UV or X-rays).

intaglio An incised carving, with the design cut below the surface of the material.

interference (of light) Cancellation of some wavelengths of light due to varying path lengths of light reflected from both surfaces of a thin layer of transparent material of RI different to the transmitting medium. With white light, non-spectral colours are produced.

interference figures Figures consisting of concentric coloured bands and dark brushes (isogyres) produced when a doubly refracting material is suitably illuminated and examined in convergent (or conoscopic) polarised light. Characteristic figures are produced for uniaxial and biaxial stones. Quartz shows a unique “bull's eye” figure.

interpenetrant twin A twin crystal in which the two portions appear to have grown from a common origin and appear to penetrate each other.

ionic bonding Bonding resulting from electrical attraction between oppositely charged ions. Each ion is surrounded by ions of opposite charge so that the whole crystal structure is neutral.

iridescence The non-spectral colouration caused by interference of light passing through or reflected from a thin transparent film of different Refractive Index to the surrounding medium.

isomorphism Replacement of elements or compounds in a mineral such that no alteration takes place in the form of the crystal of that mineral. Replacement atoms or ions must have approximately the same size as those replaced. Replacement ions must have the same total ionic charge as those replaced.

labradorescence The intense flash of spectral colours in labradorite due to diffraction at alternating layers which are exsolution lamellae of feldspars of different chemical composition.

lapidary The cutting of stones other than diamond.

laser Light amplification by stimulated emission of radiation. Monochromatic, collimated and coherent electromagnetic radiation in the visible/infra red range used for cutting, drilling and engraving of diamonds.

light Visible electromagnetic radiation spanning the wavelengths 400 to 700nm. White light has an approximately equal mixture of wavelengths making up the visible spectrum.

light-field illumination Light is transmitted through the gemstone directly to the eye; inclusions are shown in relief as a contrasting dark or outlined image against a bright background.

lines of Retzius A pattern of intersecting parallel curved lines characteristic of dentine ivory. Also called “engine turning”.

lustre (reflectivity) The extent to which light is reflected from the surface of a material. Described as adamantine, vitreous, resinous, waxy, pearly, silky, metallic.

man-made stones Stones made in a laboratory or industrial environment. They may be synthetic gemstones made to simulate growth characteristics and properties of natural gemstones (e.g. synthetic corundum) or may have no counterpart in nature e.g. Yttrium Aluminium Garnet (YAG).

massive Without external crystalline shape.

metamorphic rocks Rocks formed through the alteration in the solid state of existing rocks by high temperatures and/or pressures. Gem minerals found in metamorphic rocks include garnet, iolite and epidote.

microscope An optical instrument designed to produce enlarged images of objects. Images are formed by the objective lenses and the object is viewed through eyepieces (oculars). In gemmology, a binocular microscope using dark field illumination is commonly used.

Miller Indices Indices use to define crystallographic planes. The indices are specified as the lowest common denominator of the reciprocals of the intercepts made by a crystal face on the crystallographic axes.

mineral A homogeneous substance produced by the processes of inorganic nature, having a chemical composition and physical properties which are constant within narrow limits. Its structure is usually crystalline.

Moh's Scale of Hardness The "scratch hardness" of a mineral compared with a series of minerals of known hardness i.e. talc, gypsum, calcite, fluorite, apatite, orthoclase (feldspar), quartz, topaz, corundum and diamond.

molecule The smallest particle of a substance, be it element or compound, which can exist in a free state.

monochromatic light Light containing only a very narrow waveband of electromagnetic energy. Light of wavelength 589.3nm from a sodium vapour lamp is used for refractometer measurements.

negative crystal A crystal-shaped cavity within a crystalline gemstone. The cavity usually shows a combination of forms which occur in the host crystal and may contain liquid, gas or solid.

Nicol prism A prism consisting of two sections of optically pure calcite cemented together with Canada balsam; the ordinary polarised ray is totally internally reflected while the extraordinary ray is transmitted.

opalescence A milky translucent effect in opal and opal glass. In opal potch the silica spheres are too small or too irregularly stacked to diffract light, which is then scattered from the spheres, producing a bluish-white appearance; other colours such as orange may be transmitted.

opaque Unable to transmit light.

optic character The refractive properties of minerals are described as singly refracting (isotropic), doubly refracting (Uniaxial) or doubly refracting (Biaxial).

optic axis A direction of single refraction in a doubly refractive stone. Crystals belonging to the tetragonal, hexagonal and trigonal crystal systems have one optic axis (uniaxial) whereas crystals belonging to the orthorhombic, monoclinic and triclinic crystal systems have two optic axes (biaxial).

optic sign Uniaxial gemstones have a positive optic sign if the higher numerical RI values only varies with direction and a negative sign if only the lower RI varies. Biaxial gemstones have a positive optic sign if the higher numerical RI reading varies more with direction than the lower RI reading and a negative optic sign if the lower numerical RI readings vary more than the higher RI readings.

optical indicatrix An imaginary three dimensional ellipsoid whose radii are mutually at right angles, parallel to the principal vibration directions of the polarised rays and proportional in length to the limiting values of the refractive indices for the polarised rays.

organic gem materials Materials that are valued for their appearance and are by-products of living organisms e.g. amber, pearl, coral.

ornamental stones Minerals and rocks which normally lack transparency and are valued for their colour and pattern e.g. turquoise, lapis lazuli, malachite, agate. Although they have beauty, not all of them are rare or possess durability.

parting Separation of a crystal at a plane of weakness corresponding to a secondary twin plane e.g. basal parting in sapphire.

paste Gemstone simulants made from glass (usually lead or flint glass which both have high dispersion).

pegmatite Coarse-grained, quartz-feldspar rocks formed by the consolidation of the last portion of magma. Gem minerals found in pegmatite rocks include beryl, topaz and tourmaline.

phosphorescence Emission of visible light persisting for an appreciable interval after the stimulating radiation causing fluorescence has been turned off. The light emitted during phosphorescence is of longer wavelength than the light emitted during fluorescence.

photon A quantum of electromagnetic radiation with energy proportional to the frequency of radiation.

piezo-electricity The effect shown by polar crystals in which pressure induces opposite electrical charges on opposite faces of a slab; conversely, the crystal slab may be made to oscillate in an alternating electrical field.

play of colour Change of spectral colours occurring in each grain or patch of precious opal as the angle between the illumination direction and the observation direction is changed.

pleochroism The property by which the colour of a stone varies according to the direction in which it is viewed. The effect is due to differential selective absorption of the polarised rays in a doubly refracting gemstone. The term "dichroism" is used when only two colours are noted and "trichroism" is used where three colours are noted.

polarised light When a ray of light passes through a doubly refractive crystal it is split into two polarised waves each vibrating in a single plane at right angles to each other. In uniaxial crystals, the polarised rays are called the ordinary and extraordinary rays. In biaxial crystals, the polarised rays are called alpha, beta and gamma; only two of these polarised rays can occur in a particular direction of the light ray.

polariser An instrument utilising two polarising filters set with their vibration directions crossed. It is used to determine whether a transparent gemstone is singly or doubly refracting.

polarising filter (polaroid ™) A plastic film containing oriented crystals of quinine iodosulphate or long molecules such that light vibrating in only one plane is transmitted with minimum absorption; light vibrating in other directions is absorbed.

polymorphism The ability of minerals having the same composition to crystallise in different crystal systems. "Dimorphism" is used where two species exist e.g. carbon in the form of graphite (hexagonal) and diamond (cubic).

polysynthetic twinning Repeated contact twinning in which the twin planes remain parallel.. The series of thin plates of alternating orientation is also called "lamellar twinning".

primary (mineral) deposit Minerals originally deposited (usually in a vein) as a result of geological processes.

prism spectroscope A spectroscope of the direct vision type where the spectrum is produced by dispersion of the incident light as it passes through a train of prisms.

protogenetic (inclusions) Mineral inclusions formed before the growth of the host crystal and incorporated into the host crystal e.g. rutile in quartz, biotite in emerald.

pseudomorph A mineral which has adopted a form other than its normal crystal habit by replacing and adopting the shape of a pre-existing crystal or organic structure e.g. opal replacing shell.

pyro-electricity The simultaneous development of positive and negative electrical charges at opposite edges or opposite ends of a symmetry axis in a crystal when the crystal is heated e.g. in tourmaline.

rare earth spectra Characteristic fine line absorption spectra observed in gemstones containing rare earth elements e.g. in apatite containing neodymium and praseodymium.

reflectivity (reflectance) meter An instrument measuring surface reflectivity and developed for gemstone identification on the basis of the Fresnel equation relating refractive index to reflectivity.

refraction The bending of light rays as they pass from one medium to another medium of different optical density.

Refractive Index (RI) The constant ratio between the sine of the angle of incidence in the less dense medium and the sine of the angle of refraction in the more dense medium for any two media in optical contact. Note: since the ratio varies according to the wavelength of light used, yellow sodium light is used for refractive index measurement and air as the less dense medium. The RI is also equal to the ratio of the velocity of light in the two media.

refractometer An instrument designed to measure Refractive Index, birefringence and optic character of gemstones.

scintillation The multiple and alternating reflections or twinkling of light from the facets of a polished gemstone when there is relative movement between the observer and the light source or gemstone.

secondary (mineral) deposit A mineral deposit which has formed through alteration of a primary mineral deposit by chemical means e.g. malachite, rhodochrosite.

secondary twinning Lamellar or polysynthetic twinning produced in a crystal due to pressure applied after its original formation e.g. in sapphire and ruby.

sedimentary rocks Clastic sediments formed of fragments of other rocks transported from their sources by wind, water or ice, and usually deposited in water OR non-clastic sediments formed by chemical precipitation from solution or by biological processes.

selective absorption (of light) Absorption of certain wavelengths of light resulting from excitation of outer orbital electrons in the substance; the colour of the substance results from the wavelengths which are transmitted or reflected and not absorbed.

sheen A special visual phenomena observed in gem materials due to reflection of light from the internal structure of the stone.

simulant (imitant) A material, either natural or synthetic, having a superficial resemblance to a gemstone which it imitates. The properties of the simulant are usually distinctly different to the gemstone imitated e.g. zircon imitating diamond, blue glass imitating blue sapphire.

skull melting A technique used to synthesise cubic zirconia. Zirconia powder is contained within a skull of water cooled fingers, then heated by radio frequency radiation. The molten material confined within a crust of itself is then slowly cooled , forming columns of crystals.

Snell's Laws of Reflection
1) “The angle of incidence of a light ray striking a flat reflecting surface is equal to its angle of reflection”.
2) “The incident ray, the reflected ray and the normal (at the point of incidence) all lie in the same plane”.

Snell’s Laws of Refraction
1) “When a light passes from one medium into another, there exists a definite ratio between the sines of the angle of incidence and the angle of refraction; this ratio is dependent only on the two media and the wavelength of the light”.
2) “The incident ray, the refracted ray and the normal (at the point of incidence) are all in the same plane”.

Specific Gravity (SG) The weight of a body compared with the weight of an equal volume of pure water at 4 degrees Celsius.

“spot” RI An approximate RI measured with a refractometer using the distant vision technique. The scale reading taken is when the image of the liquid contact between the gemstone and refractometer window appears half dark and half light.

Steno’s Law (Law of Constancy of Interfacial Angles) “In all crystals of the same substance, the angles between corresponding faces have the same value (when measured at the same temperature)”.

striations Parallel markings on the surface of crystals resulting from oscillating growth between two crystal forms e.g. horizontal striations on the prism faces of quartz crystals.

symmetry axis An imaginaiy line passing through the centre of the crystal, about which the crystal may be rotated to present the same appearance twice or more during one complete rotation.

symmetry plane An imaginary plane dividing a body into two halves, each of which is the exact inverse counterpart (or mirror-image) of the other.

syngenetic (inclusions) Mineral inclusions formed at the same time as the host crystal and enclosed within it e.g. healed fractures in quartz, olivine in diamond.

synthetic (gemstones) Gemstones produced under laboratory or industrial conditions from the same chemical ingredients as the natural gems and having the same major properties as their natural counterparts e.g. synthetic emerald, synthetic ruby.

tenacity (toughness) The ability of a mineral to absorb energy without breaking or cleaving. Terms used are brittle, sectile, malleable, flexible, elastic.

thermal conductance (thermal conductivity) tester An instrument designed to distinguish diamond from other materials on account of the higher thermal conductivity of diamond. Note: some instruments may measure the thermal inertia of the stone.

total internal reflection Reflection of light travelling from an optically denser to an optically rarer medium when the critical angle of incidence is exceeded.

transition elements A group of eight metallic elements with Atomic Numbers 22 to 29 which have variable valency; absorption of light occurs as a result of unpaired electrons in a subshell. These elements - titanium, vanadium, chromium, manganese, iron, cobalt, nickel and copper are largely responsible for colour in idiochromatic gemstones.

translucent Allows transmission of light but does not show a clear image of an object.

transparent Transmits a clear, undistorted image of an object. Most faceted gemstones are transparent.

trigon A triangular growth depression on the octahedral face of a diamond crystal. The points of the trigon are oriented towards the straight edges of the octahedral face.

twin crystal A crystal consisting of two or more portions of the same mineral crystal which have grown in such a way that some crystallographic plane is common to the parts of the twin. There are two main types of twin crystal - contact twins or interpenetrant twins.

two or three phase inclusions Irregular drop shape or crystal shape cavities containing two phase remnants (liquid-gas, gas-solid, solid-liquid) or three phase remnants (gas-liquid-solid).

ultraviolet (UV) light Electromagnetic radiation with wavelengths shorter than the range of visible light, from 380nm to about 20nm. Ultraviolet light of wavelength 253.7nm (short wave UV) or 365nm (long wave UV) is used to determine the fluorescent response of gemstones.

unit cell The smallest possible spatial unit which can possess all the physical, chemical and geometrical characteristics of the mineral as a whole.

valency The combining power of an ion; the number of electrons which the atom needs to lose or gain to obtain stable outer shells of electrons.

varieties The variations in a mineral species (and differing appearance) due to trace elements or minor structural differences e.g. quartz varieties include rock crystal, smoky quartz, citrine, amethyst.

Verneuil (flame fusion) process A high temperature process used to produce synthetic gemstones. Ingredients in powder form (e.g. aluminium and chromium oxides for synthetic ruby) are melted in an oxy-hydrogen flame forming a pointed cylindrical “boule”.

Wood’s Filter A glass filter, containing cobalt and nickel, which separates the 366nm line emitted by a mercury discharge lamp from the visible emission spectrum. The filter is used in construction of a Long Wave Ultra-Violet lamp.

X-rays Highly penetrating electromagnetic radiation with wavelengths from 20nm down to less than 0.00 Inm. X-rays are used in gemmology for determining crystal structure by X-ray diffraction, in testing X-ray fluorescence of gemstones and in distinguishing natural from cultured pearls.