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    Cabochon a style of stone cutting with a dome-shaped top.

    Cacholong a bluish-white porcelain-like variety of opal.

    Cacoxenite quartz, usually amethyst, with radiating crystals of cacoxenite.

    Cairngorm a brownish-yellow variety of crystalline quartz found in the Cairngorm Mountains, Scotland.

    Calamine the name used in some English mineralogical books for the zinc carbonate Smithsonite, which see. In other works this name is used for a zinc silicate.

    Calcite R.I. 1-486 --- 1 651 S.G. 2-71 H. 3 Trigonal Colour, colourless and white, sometimes with grey yellow, blue, red, brown or black tints. Marble is a massive calcite, a fibrous form being known as "Satin-spar" and stalagmitic forms with well-marked banding are used for small objects and ornaments and are known under the MISNOMER ( COMMERCIAL LABEL )s: "Algerian onyx", "Mexican onyx" and "Oriental alabaster". Calcite of optical quality, known as Iceland spar, is used in the construction of Nicol prisms and in the dichroscope.

    Calcium titanate This compound has been synthesized and may appear on the market as a new gemstone. The material is colourless, orthorhombic, but near cubic, in crystallization. The hardness is 6 to 6l, the density 4-05 and the refractive index 2-40.

    Calibre-cut the term applied to stones cut to special shapes. Usually trap cut in style with sharp angular corners. The small square stones used for "eternity" rings are sometimes called calibre.

    Californite a massive variety of idocrase which simulates jade. See Idocrase.

    Callaite also Callaica, Callaina, and Callais earlier mineralogical names for turquoise, see Turquoise.

    Calorescence the term applied to the phenomenon exhibited by certain minerals which, when irradiated with heat rays produce visible light. See also Thermoluminescence and Luminescence.

    Cameo the term used to designate those stones, generally composed of two differently coloured layers, in one of which a raised figure or design is cut, while the layer of the second colour forms a background. Agates and certain sea shells are usually the materials used.

    Canada balsam a resin obtained from a species of fir and used as a mountant for microscopic specimens. R.I.= 1 53.

    Canada moonstone a name applied to the peristerite variety of feldspar. See Peristerite.

    Cancrinite R.I. approx. 1-51 S.G. 2-42 to 2-50 H. 5 to 6 Hexagonal Massive opaque yellow or orange. Gem locality is Canada.

    Canutillos the name by which good quality emerald crystals are locally known by the miners working the Colombia emerald mines.

    Cap-cut a fashioned stone in which the facets are irregular and haphazard. See Bastard-cut.

    "Cape emerald" a MISNOMER ( COMMERCIAL LABEL ) which has been applied to the mineral prehnite found in South Africa. See Prehnite.

    "Cape ruby" a MISNOMER ( COMMERCIAL LABEL ) for pyrope garnet found in association with diamond in South Africa.

    Cape stones a classification of gem diamond sub-divided into: fine silver Cape, silver Cape light Cape, Cape, dark Cape. This group is classified next in order to blue-whites and whites.

    Carat (precious metals)a term used to express the fineness of gold used in jewelry. It may be better understood as a twenty-fourth part, thus 9 carat gold contains 9 parts of pure gold and 15 parts of alloy, likewise 22 carat gold contains 22 parts of pure gold and 2 parts of alloy. The term is sometimes spelled Karat.

    Carat weight the unit of weight for diamonds and gemstones. It is defined as one-fifth of a gram (200 milligrams=0 200 gram). It became legal standard on the 1st of April, 1914, and was and frequently still is, known as the metric carat. The old London carat weighed 0.20530 gram, was not a legal standard and did not conform to the carat weight in other parts of the world, which varied in different countries from 0-1885 gram to 0-2135 gram.

    (1) see Carbonado.
    (2) black inclusions in diamond, often referred to as carbon marks.

    Carbonado an opaque, black, tough and compact variety of diamond found in Brazil, specific gravity being between 2-9 to 3-5. It is used for drilling Bits for deep boring and for slow running abrasive wheels.

    Carbon disulphide A liquid R.I. 1 63.

    Carborundum H. 9.5 S.G. = 3-17 a synthetic product made by heating coke and sand in an electric furnace. Used as an abrasive.

    Carbuncle a name for almandine garnet which has been cut en-cabochon.

    Carnelian see Cornelian.

    Cascalho the native name for the diamond-bearing gravel of Brazil.

    Casein a synthetic substance made from the albumen of milk and used occasionally as an imitation of amber and tortoiseshell and some ornamental stones. R.I. 1-55 to 1-56 S.G. 1-32 to 1-39 (usually 1-32 to 1-34.

    Cassia oil a vegetable oil akin to cinnamon oil. R.I.= 1 60.

    Cassiterite R.I. 1-997 --- 2- 093 S.G. 6-8 to 7-1 H. 6 to 7 Tetragonal Colours, red, brown, black and yellow Localities, Cornwall, Bohemia and Saxony.

    Castor oil a pale yellow oil obtained from the seeds of ricinus communis and used as an immersion medium in certain refractive tests. R.I.= 1-48.

    Cat's-eyes stones which, when cut en-cabochon, show a wavy changeable band of light across the dome. The phenomenon is known as "chatoyancy", and is often observed in quartz, chrysoberyl and tourmaline. See also Chatoyancy, Hawk'seye and Tiger's-eye.

    Cedar wood oil a vegetable oil used in immersion refractive index tests. R.I. = 1-51.

    Celestine (Celestite) R.I. 1-62 --- 1-63 S.G. 3-97 to 4-00 H. 3 to 3.5 Orthorhombic Colourless to bluish Locality U.S.A.

    Celluloid a thermo-plastic material made from a nitrocellulose base, sometimes used as an imitation of amber, etc. Two types:
    A-Ordinary celluloid (cellulose nitrate). R.I. 1-495 to 1-51 S.G. 1-36 to 1-80 (usually in the clear types 1-36 to 1-42.
    B-Safety celluloid (cellulose acetate). R.I. 1490 to 1505 S.G. 1-29 to 1-80 (usually 1-29 to 1-40)

    Cerussite R.I. 1-80 --- 2-1 S.G. 6-5 H. 3.5 Orthorhombic White, grey, green, blue and black. World wide occurrence.

    Ceylon diamond a MISNOMER ( COMMERCIAL LABEL ) for colourless zircon.

    Ceylonese chrysolite a MISNOMER ( COMMERCIAL LABEL ) for greenish-yellow tourmaline.

    Ceylonese peridot a MISNOMER ( COMMERCIAL LABEL ) for yellowish-green tourmaline.

    Ceylonite a dark green, almost opaque spinel, rich in iron sometimes used in jewelry. An alternative name for this variety is Pleonaste, see Spinel.

    Ceylon opal a MISNOMER ( COMMERCIAL LABEL ) for moonstone.

    C.G (Certified Gemmologist) suffix used by gemmologists who have qualified by the examination of the Gemmological Institute of America.

    Chalcedony a micro-crystalline variety of quartz , with the following varieties:Chalcedony common translucent with a white or bluish colour, green when containing chromium.

    Cornelian translucent flesh-red.
    Sard brownish red.

    Chrysoprase translucent apple green.
    1-Plasma dark green with white or yellowish spots.
    2-Bloodstone or Heliotrope dark green with scattered spots of red jasper.
    3-Agates chalcedony where the colour is variously distributed, generally in parallel layers.
    4-Banded agate colours in parallel bands.
    5-Eyed-agate bands having a circular arrangement.
    6-Fortification agate bands are angularly arranged.
    7-Moss agate or Mocha stone containing dendritic inclusions. Agatised wood chalcedonic pseudomorph after wood.
    8-Onyx similar to agate except that the bands are straight. Cameos are usually cut from these. Onyx, like all chalcedonies, can be stained, the black onyx nearly always has been so treated.
    9-Sardonyx as onyx except that instead of the colours being black and white they are brownish red and white.
    10 Jasper an impure variety of micro-crystalline quartz, opaque reds and browns also greyish blue and greens. Riband jasper is striped.
    11 Hornstone a grey impure form which is sometimes stained to imitate lapis-lazuli. (This shows red under the colour filter whereas true lazurite does not.)
    The refractive indices and specific gravity of Chalcedony are but slightly lower than for Quartz.

    Chatham emerald (Chatham Cultured Emerald: Chatham Created Emerald) names applied at various times to an American synthetic emerald, grown by C. F. Chatham.

    Chatons paste (glass) stones backed with a reflecting foil.

    Chatoyancy (cat's-eye effect) is due to the reflection of light from fine fibres or fibrous cavities within the stone. The wavy band of light seen across the stone being at right angles to the direction of the fibres. To show this best stones must be cut en-cabochon. See also Asterism.

    Chatter marks see Fire marks.

    Chelsea colour filter

    Chemical composition the composition of a molecule of a substance, which may be an element or a combination of different elements in quantities which must obey certain definite chemical laws.

    Chemical composition of gem minerals See Chart

    Chemical elements matter composed of only one chemical type, and which thus cannot be decomposed into simpler substances by chemical means. See Chart

    Chessylite see Azurite.

    Chiastolite a variety of andalusite (which see) containing carbonaceous inclusions in the form of a cross

    Chicot pearls an alternative name for blister pearls. See Blister pearls.

    Chlorastrolite a greenish fibrous mineral related to prehnite. S.G. 3-2 H. 5 to 6 Colour, chatoyant green and white Locality, Lake Superior (U.S.A.).

    Chloromelanite a dark green nearly black ferruginous variety of jadeite. S.G. = 3-4. H. - 6.5 to 7.

    Chlorospinel see Spinel.

    Chondronite R.I. 1-60 --- 1-63 S.G. 31 H. 6.5 Monoclinic Yellow, red, brown Localities, Sweden and U.S.A.

    Chrome chalcedony green chalcedony coloured with chromium. Found in Rhodesia. Not to be confused with chryosprase which it resembles.

    Chrome diopside a bright green diopside found in association with diamond in South Africa.

    Chromite S.G. 4-3 to 4-6 H. 5.5 Cubic, Colour iron-black to brownish-black Localities, U.S.A., etc.

    Chromium oxide a green powder used as a polishing agent, See Green rouge.

    Chryselephantine the name applied to objects of art composed or overlaid, partly with gold and partly with ivory.

    Chrysoberyl R.I. 1-742 --- 1-749 to 1-75 --- 1-757 S.G. 3-68 to 3-78 H. 8.5 Rhombic Colour, greenish-yellow, greenish chatoyant (Cymophane or Cat's-eye), emeraldgreen in daylight and red in artificial light (Alexandrite) Localities, Brazil, Ceylon, Ural Mountains (Russia), and Rhodesia.

    Chrysocolla a hydrous copper silicate R.I. 1-50 S.G. 2-1 to 2-2 H. 2 to 4 Amorphous Colour, green and greenish-blue, Localities, Ural Mountains (Russia), Chile and Arizona (U.S.A.). Often impregnating) quartz.

    Chrysolite an ancient name applied to various kinds of yellow and greenish-yellow stones. A name which is better discontinued.
    Chrysolite, Water a MISNOMER ( COMMERCIAL LABEL ) for moldavite.

    Chrysoprase apple-green chalcedony.

    Cinnamon oil an aromatic oil used in certain refractive index tests. R.I. = 1-59.

    Cinnamon-stone brownish-red hessonite garnet.

    Circular polarisation the peculiar property of quartz, among gemstones, of rotating the plane of polarisation of a ray of light passing parallel to the optic axis, and showing an interference figure in convergent polarised light, in which the arms do not meet at the centre, the four arms stopping at the innermost ring.

    Citine yellow quartz, see Quartz.

    Clam pearls pearls obtained from the clams such as the quahog or hard clam (Venus mercenaria) and the giant clam (Tridacna gigas). Density varies from 2-20 to 2-66.

    Cleavage the tendency of a crystallised mineral to break along certain definite directions producing more or less smooth surfaces.

    Cleavage name applied to diamond crystals showing many flaws, or to broken fragments of crystals.

    Cleaving the method of dividing a diamond crystal into two or more pieces by splitting the stone through the grain (cleavage direction).

    Clerici's solution

    Clmozeisite An epidote containing less than 10% of the iron molecule R.I. 1-72 --- 1-734 S.G. near to 3- 37. It is a lighter green than epidote.

    Close goods are whole diamond crystals which contain no flaws.

    Clove oil an aromatic oil used in certain refractive index tests. R.I. = 1-54.

    Coated stones diamond crystals having a coat of green, brown or yellow colouring removable by cutting.

    Cobalt glass a glass coloured blue by cobalt oxide. Often used in the production of imitation gems, this glass is characterised by a typical absorption spectrum.

    Cobaltite S.G. 60 to 64 H. 5.5 Cubic Colour, silver-white Localities, Scandinavia, U.S.A. and England.

    Cohesion the name given to the force of attraction existing between the molecules of one and the same body in consequence of which they offer a resistance to any influence tending to separate them.

    Colemanite R.I. 1-58 --- 1-61 S.G. 2-42 H. 4.5 Monoclinic Colourless and white Locality, U.S.A.

    Collet, Collette alternative names for culet which see.

    Collimator the lens system in certain optical apparatus used to parallelise the incident light rays. See also Spectrometer.

    Colloid the term applied to a liquid or solid compound of one substance in fine particles of ultra-microscopic size diffused through another.

    Colorado jade a MISNOMER ( COMMERCIAL LABEL ) for amazonite. See Feldspar.

    Colorado ruby a MISNOMER ( COMMERCIAL LABEL ) for pyrope garnet found in Colorado, U.S.A.

    Colorimeter, Diamond see Diamond colorimeter.

    Coloriscope a Swiss instrument for the colour grading of diamonds.

    Colour dispersion See Chart

    Coloured diamonds diamonds having a different shade of colour, termed Fancy diamonds, such as red, pink, blue, mauve, green, canary-yellow and brown. (See note on irradiated diamonds)

    Colour filters coloured films or glasses used to filter out certain colours of the spectrum.

    Colours of gemstones

    Composite stones

    Conchoidal fracture --- 2-6 S.G. 5-9 to 6-1 H. 2.5-3 Monoclinic Hyacinth red Localities, Tasmania, Russia, Romania, Brazil and U.S.A.

    Crocus (Crocus martis) a polishing powder produced from an iron oxide.

    Cross-cut See scissors cut.

    Cross facets the name applied to eight of the small three-sided facets around the girdle edge on the crown, which in the case of a modern circular stone have the same size and shape as the eight skill facets adjacent. In older oval-shaped stones these facets are the eight larger of the 16 edge facets. In modern nomenclature the eight cross and the eight skill facets are combined as 16 half or break facets. An alternative name for cross facets is skew facets.

    Cross stones fancy name for the twinned crystals of staurolite. Also known as "Fairy stone".

    Cross work the name applied to the first operation in grinding a brilliant-cut stone, consisting in grinding the table and four main facets

    Crown that part of a cut stone which lies above the girdle, or setting edge. In the brilliant-cut stone it has the table facet and 32 surrounding facets. An alternative name is the bezel.

    Crown glass a classification or family of glasses which do not include lead oxide in their composition. In general they have lower constants than for the more highly dispersive "flint" or "lead" glasses.

    Crypto-crystalline the term used to describe material made up of an aggregate of sub-microscopic crystals.

    Crystals solids possessing a certain definite internal atomic structure, which is identical in the case of crystals of any one species. This definite arrangement directly influences the geometrical form and the physical and optical properties.

    Crystal axes imaginary lines of reference running through the ideal crystal and intersecting in the centre at a fixed point, termed the origin. They are reference lines from which can be measured the relative positions of the various faces.

    Crystal faces the flat surfaces of geometrical outline which form the bounding surfaces of crystals. In the case of some diamonds these surfaces are curved.

    Crystal lattice the three dimensional array of points (atomic positions) in space at which the pattern repeats itself. There are only 14 possible variations of the crystal lattice.

    Crystalline material any material which shows by physical and optical means the regular arrangement of its internal atoms.

    Crystallography the study of crystals and their structure.

    Crystal, Rock see Quartz.

    Crystal systems see Chart.

    Crystolon trade name for an abrasive made of silicon carbide.

    Cube a solid of six square faces with all its angles right-angles. The fundamental crystal form of the cubic system.

    Cubic system one of the crystal systems, see Chart

    Cubo octahedron a crystal form combining the cube and the octahedron.

    Culet the small facet at the base of the pavilion of a brilliantcut stone parallel to the table facet. Its main function is to prevent splintering but it is often omitted in modern cut stones.

    Cullinan diamond also known as the Star of Africa, is the largest diamond ever found. Found at the Premier mine in the Transvaal on 25th January, 1905, the rough stone weighed 3,106 carats. From this magnificent stone two important diamonds were cut one, a pendeloque brilliant weighing 530-2 carats is the largest cut diamond in the world and is mounted in the Royal Sceptre of the British Regalia the other, a square brilliant weighing 317-4 carats is set in the Imperial State Crown. These two stones are known respectively as Star of Africa No. 1, and Star of Africa No 2.

    Cultured pearl a pearl produced by the insertion in the pearl oyster of an artificial nucleus, usually mother-of-pearl, and the deposition of nacre thereon by the mollusc.

    Cuprite R.I. 2 85 S.G. 5 85 to 6 15 H. 4 Cubic Red Widespread occurrence.

    Curvette see Chart.

    Cut-Cornered triangle the name applied to a trap-cut stone in which the outline is that of a triangle with two of the corners bevelled off. See Cuts of stones, see Chart.

    Cuts of Stones see Chart

    Cuttable rough a name which has been applied to all diamonds suitable for manufacturing into ornamental or gem stones.

    Cutting the process of cutting gemstones on revolving diamond charged grinding wheels.

    Cyanite alternative spelling for kyanite, which see.

    Cyclotroned diamonds Diamonds which are coloured green by bombardment with atomic particles which have been accelerated to a high speed by a cyclotron. After subsequent heat treatment the stones become yellow or brown. The colouration is only skin deep and the stones are not radioactive.

    Cymophane see Chrysoberyl.

    Cyprine see Idocrase.

    Cyst pearls pearls formed within the tissue of the mollusc itself. These pearls are the most perfect.