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    "Iceland agate" a MISNOMER ( COMMERCIAL LABEL ) for brownish or greyish obsidian.

    Iceland-spar see Calcite.

    Ideal cut the name given to a diamond cut to the correct Proportions for maximum brilliance.

    Idiochromatic minerals those in which the colouring is due to some essential constituent, the colour being constant and therefore of assistance in identification, e.g., Malachite. See also Allochromatic minerals.

    Idocrase R.I. 1-702 ---1-706 to 1-72 --- 1-732; S.G. 3-35 to 3-45; H. 6.5; Tetragonal; Colours, yellow, green, brown, blue (containing copper and called Cyprine), compact green (Californite); Localities, Italy, Siberia, Norway and U.S.A. Alternative name, Vesuvianite.

    Igneous rocks solidified molten magma of mineral matter, mostly of complex composition.

    "Igmerald" trade name given to the synthetic emerald produced in Germany; a true synthetic beryl having constants and colour near those of natural emerald. May be detected by certain characteristic internal markings. Not now made.

    Illam name given to the gem-bearing gravel of Ceylon.

    Imitation stones a man-made stone that is made to resemble a genuine stone but that is not chemically or structurally the same.

    Immersion liquids a series of liquids that are of different specific gravities so that a stone can float or sink to determine if it is heavier or lighter than the liquid.

    Imperial jade the name given to the apple-green or emeraldgreen translucent jadeite. A highly prized colour.

    "Imperial mexican jade" a MISNOMER ( COMMERCIAL LABEL ) for green-dyed calcite. Inanga; Maori name for the grey variety of nephrite.

    Inca rose a fancy name for Rhodochrosite.

    Inca stone iron pyrites.

    Inclusions general name for crystals and liquid- or gas-filled cavities enclosed in a "host" mineral. Such inclusions are often highly characteristic of the mineral concerned and are the surest basis for the distinction between natural and synthetic stones.

    Index of refraction The relative amount that light is bent as it enters a gem. Used as a way of identifing gems.

    "Indian emerald" a MISNOMER ( COMMERCIAL LABEL ) for green-dyed crackled quartz (rock crystal).

    "Indian jade" aventurine quartz, which see.

    "Indian topaz" a MISNOMER ( COMMERCIAL LABEL ) for yellow corundum.

    Indicators pieces of glass or small fragments of gem-stones of known specific gravity used to indicate the approximate density of heavy liquids.

    Indicolite see Tourmaline.

    Industrial diamonds diamonds used for industrial purposes, such as deep well boring, turning tools for metals and plastics, truing abrasive wheels, dies for fine wiredrawing, hardness indenters, and as powder for grinding and polishing metals.

    Infra-red (Heat rays) electro-magnetic radiations of wavelengths between 7,900 Angstrom units and 10,000,000 Angstrom units, that is beyond the visible red. These radiations produce in certain minerals a luminescence. See Thermo-luminescence.

    Inorganic not produced by vital processes.

    Interfacial angle the angle between any two faces of a crystal. In crystallography, the interfacial angle is the angle between the normals, or perpendiculars, to the two faces.

    Interference figures the figures seen when anisotropic minerals are viewed in convergent polarised light. They afford useful information concerning the optical characters.

    Interference of light where two rays travelling in the same path, but out of phase, mutually interfere with one another causing either total extinction of light or predominance of one or more colours. This is the cause of the play of colour in opal and thin films.

    Internal strain local straining of the atomic structure due to pressure caused by solid inclusions within the crystal, or through rapid cooling, as in some glass and synthetic gems. This strain gives rise to anomalous double refraction.

    Interpenetrant twins where two or more crystals penetrate each other along a common twinning direction, common in fluorspar.

    Iodobenzene a liquid having a value in certain refractive tests. R.I.= 1 62.

    Iodo-naphthalene (a type) a liquid having a refractive index of 1704 which is suitable for use as a contact liquid for the Rayner spinel type refractometer.

    Iolite R.I. 1-53 --- 1-54 to 1- 5 --- 1-55; S.G. 2-58 to 2-66; H. 7 to 7.5; Rhombic; Colour, blue (strongly pleochroic); Locality, Ceylon; Alternative names, Cordierite, Dichroite.

    Ion an electrically charged atom, radicle or molecule.

    Iridescence the prismatic colours seen in cracks and flaws in a stone, well seen in iris quartz. Due to interference of light at thin films of differing refractive index. The surface tarnish in haematite, etc., is due to this effect.

    Iridium Ir. S.G. 22 41; one of the platinum group of metals having little application in jewelry except in the hardening of gold and platinum.

    Iris rainbow quartz; see Quartz.

    Irish diamond a MISNOMER ( COMMERCIAL LABEL ) for rock crystal.

    Irradiated diamonds diamonds which are artificially coloured by bombardment with atomic particles or gamma rays. See also Cyclotroned diamonds, Radium-treated diamonds, Pile-treated diamonds and Electroned diamonds. Also called Atomic Diamonds.

    Irregulars the term used for gem diamond crystals of irregular shape, or long shape.

    "Isle of Wight diamonds" a MISNOMER ( COMMERCIAL LABEL ) for rock crystal.

    Isomorphous replacement where one element in the chemical composition of a mineral is replaced by another element having the same valency without sensibly altering the structure and form of the crystal. This causes wide variations in the physical properties, e.g., Garnet.

    Isotropic the term used to denote all materials which are singly refractive to light, that is, light travels through them as one ray and having the same velocity and character in all directions. All crystals of the cubic system and all amorphous materials are isotropic. See also Anisotropic.

    Ivory (dentine) an organic substance from the tusk of the elephant, walrus, hippopotamus, narwhal, etc. S.G. 1-70 to 1-98; R.I. 1-54; H. 2.5.

    Ivory (vegetable) the hard white kernel of the nut of certain palm trees (in particular that of Phytelephas Macrocarpa from South America) ; S.G. 1-38 to 1-42 ; R.I. 1-54 ; H. 2.5.