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Gahnite R.I. 1-805; S.G. 4-40; H. 7.7 to 8; Cubic; Dark green; many localities.


Gahnospinel blue spinel containing considerable proportion of zinc. Found in Ceylon. S.G. up to 4-06; R.I. up to 1-753.


Garnet an isomorphous series of minerals (gem-stones) represented by the formula R"3R"'2, where R" may be any of the bivalent metals magnesium, calcium, manganese or iron, while R"' stands for a trivalent metal, aluminium, ferric iron or chromium. All garnets crystallise in the cubic system. The garnets are divided into two series; the "pyralspite series" which consists of pyrope, almandine and spessartite; and the "ugrandite group" consisting of uvarovite, grossular and andradite.
(1) Grossular; R.I. 1 742 to 1 748; S.G. 3 55 to 3 67 ("Transvaal Jade", 3-42 to 3-72; H. 6.5 to 7;
(A) ) Hessonite Colours, brownish yellow (Cinnamon-stone) reddish orange, transparent green massive green ("Transvaal Jade"), and other colours; Localities, Ceylon, Switzerland and South Africa.
(2) Pyrope; R.I. 1-74 to 1-75; S.G. 3-68 to 3-80; H. 7.5; Colour, red; Localities, South Africa, Bohemia, Arizona, etc.
(3) Spessartite; R.I. 1-79 to 1-81; S.G. 3-90 to 4-20; H. 7.5; Colours, brown, orange-red and hyacinthred; Localities, Bavaria, Ceylon, Tyrol and U.S.A.
(4) Almandine; R.I. 1-75 to 1-82; S.G. 3-80 to 4-20; H. 7.5; Colours, deep red, violet-red and black; Localities, Australia, India, Ceylon and North and South America.
(5) Four-rayed asterism sometimes seen.
(6) The lower range given here for the almandine is sometimes described as the pyrope-almandine series.
(7) Uvarovite; R.I. 1-84 to 1-85; S.G. 3-41 to 3-52; H. 7.5; Colour, bright green; Locality, Russia.
(8) Andradite; R.I. 1-82 to 1-89; S.G. 3-80 to 3-90; H. 6.5; Colours, green (Demantoid, R.I. 1-88 to 1-89; S.G. 3-83 to 3-85), yellow (Topazolite) and black (Melanite); Localities, Russia, Saxony and Italy.
(9) Rhodolite; a violet variety in composition lies between pyrope and a]mandine in the ratio of 2:1. R.I. 1-76; S.G. 3-84; H. 7.25; Locality, Macon County, North Carolina (U.S.A.).


Garnet-topped doublets composite stones made with a garnet top on a glass base.


Garnet-type synthetic stones crystals which have been grown synthetically. The most important as gemstones are the yttrium aluminium garnets (Diamonair).


Gauge, Diamond see Diamond gauge.


"Garnet, Synthetic" a MISNOMER ( COMMERCIAL LABEL ) for the synthetic dark red corundum made to represent the garnet colour.


Gel a jelly, a colloid.


Gem-cut diamond any diamond cut as a gem, it is a United States definition for export purposes.


Gemmology the science which deals with precious minerals and other materials used for personal adornment, and for objets d'art.


Gemolite an American instrument used for observing imperfections and internal features in diamonds and coloured gemstones. The instrument functions similarly to the Diamondscope, which see.


Gemological Institute of America a school that is run by the jewelry industry for training in jewelry related fields. Courses are offered by correspondance and at several fixed campuses.


Gemolux a Swiss instrument incorporating a lighting device for the examination of stones.


Gem peg see Jamb peg.


Gems, Artificial see Artificial gems.


Gems, Imitation see Imitation gems.


Gem-stick the appliance used by the lapidary for holding the rough stone when cutting the facets. Of a penholder-like form with a metal collet at the end filled with cement into which the material to be cut is fixed, leaving exposed just sufficient to cut either the front or the back facets. See Jamb peg.


Geodes crystal-lined cavities found in rocks.


Geology the science which treats of the structure and mineral constitution of the earth.


German lapis jasper or chert stained blue and used to imitate lapis-lazuli.


Ghost crystals the so-called "phantom" or "ghost" crystals of quartz appear to contain one or more crystals similar to it. The effect is due to interruption of growth.


GIA abreviation for the Gemological Institute of America.


Gilson synthetic emerald a synthetic emerald produced by the firm of Pierre Gilson, of France.


Girasol see Opal.


Girdle name applied to the outer edge of a cut stone. It is the line of junction of the top (crown) and the base (pavilion). Also termed the setting edge.


Glycerine a sweet colourless viscid liquid obtained from oils, fat, etc. The refractive index is 1-47. See Chart 131.


Gneiss a banded metamorphic rock derived either from igneous rocks (Ortho-gneiss) or sedimentary-rocks (Paragneiss).


Goethite Hydrated iron oxide, Fe203H2O, is often an inclusion in oligoclase feldspar giving rise to the variety sunstone.


Gold the precious metal of yellow colour used as a setting for precious gems. See also Carat and Tests for precious metals, Chart 219. See also Yellow gold, Red gold, Creen gold, Blue gold, etc.


"Gold Buss" another name for Goldstone.


Gold opal an alternative name for fire opal.


Gold quartz milky quartz containing particles of native gold. This is sometimes cut and polished for jewelry.


Goldstone a paste (glass) imitation of reddish-brown aventurine made by the inclusion of copper crystals in the glass. See Aventurine glass.


Goniometer an instrument for the measurement of crystal angles.


Goshenite colourless beryl, which see.


Grain a unit of weight (Troy and Avoirdupois) see Chart 224; the pearl grain is one fourth of a carat, see Chart 224.


Gramme (Gram) unit of weight of the metric system. Some ornamental stones are sold by the gramme.


Granite A granular igneous rock containing principally quartz, feldspar and mica. It is sometimes cut and polished for small jewelry articles.


"Green garnet" a name at one time incorrectly used for enstatite.


Green gold gold in which a greenish colour is induced by considerably increasing the proportion of silver in the 14 ct. quality. Several shades are obtained by careful manipulation of the percentages. Used as a contrast.


Green rouge a polishing powder consisting of chromium oxide.


Greenstone see Nephrite.


"Green stone" a MISNOMER ( COMMERCIAL LABEL ) sometimes used for chlorastrolite. See Chlorastrolite.


Grey gold a grey colour induced in a gold alloy by the addition of iron or steel. It has a limited application.


Grit an alternative name for diamond powder.


Grossular see Garnet.


Gypsum R.I. 1-52 --- 1-53; S.G. 2-2 to 2-4; H. 2; Monoclinic; Colour, white (Satin spar and Alabaster); Localities, England, U.S.A. and Italy. See also Selenite.