GADOLINIUM GALLIUM GARNET :
Synthetic stone.
Oxide of gadolinium and of gallium fabricated mainly as a substitute for diamond.
Gd₃ Ga₅ O₁₂  
Physical and optical properties :
- Colours : colourless, yellowish to brownish.
- Hardness : 6.5
- S.G. : about 7.O
- R.I. : about 2.0
- Dispersion : 0.38 (quite close to the  dispersion of diamond : 0.44).
- Under U.V. : weak orange to yellowish fluorescence under L.W.U.V. and strong orange fluorescence under S.W.U.V.

GAGAT :
German Term for Jet.

GAHNITE :
Variety of green Spinel with a main zinc composition.
ZnAl₂0₄
Rare gemstone.
Physical and optical properties :
- Colours : dark green, green, yellowish green, blue-green to blue, blackish
- Transparency : transparent to opaque
- Hardness : 7.5
- S.G. : 4.40 to 4.65 (superior to " normal "spinel).
- R.I. : 1.805 (+ 0.013 / – 0.014) (superior to " normal "spinel).
- Crystal system : cubic
Inclusions :
Negative octahedron crystals with or without libellae or tension crack; hexagonal crystals; mica; two-phase inclusions.
Confusions :
Natural corundums and synthetics, doublets.
Synthetics :
Synthetic Gahnite exists : transparent colourless ; RI : 1.805 ; SG : 4.40 ; hardness : 7.5 to 8
Imitations :
Spinel synthetic (recognition : S.G. : 1.72 to 1.73 ; inclusions of bubbles possible ; often strong fluorescence under SWUV)

GAHNOSPINEL :
(Mg.Zn)Al₂O₄
Physical and optical properties :
- Colours : very dark blue, blue-green, green, greyish-green, pale violet.
- Hardness : 7.5
- S.G. : 3.98 to 4.4
- R.I. : 1.748 to 1.805

GAIR :
Burmese Term designating a big opaque ruby.

GALALITE :
Caseine, a plastic material, coloured, serving to imitate Amber, Coral, Jet, Ivory.

GALANITE :
Imitation of Amber.

GALMEI :
Synonym for Calamine or Smithsonite.

GALLIZINITE :
Synonym of Rutile.

GARAMANTICUS :
Very ancient name. Synonym of Garnet.

GALACTITE :
By certain ancient authors : a term given to Emerald with white veins.

GARNET :
Family of ortho-silicates with general formula R''₃.R'''₂(SiO₄)₃
This family comprises as gemstones:
- Calcium-aluminium Garnet: grossular.
- magnesium-aluminium Garnet: pyrope.
- iron-aluminium Garnet: almadine.
- manganese-aluminium Garnet : spessartite.
- calcium-iron Garnet: andradite.
- calcium-chromium Garnet: uvarovite.

There exist two categories of garnets, classified according to the presence or absence of calcium.
Calcium Garnets = ugrandite garnets (uvarovite, grossulaire, andradite) with as gemstone qualities hessonite, tsavorite, melanite, topazolite and demantoid.
Alumium Garnets = pyralspite garnets (pyrope, almandin, spessartite) and as gemstones their intermediate types (pure, like all garnets they don’t exist in nature): pyrope-almandin, almandin-spessartite.

The most recurring crystalline forms are the rhombendodecahedron, the trigonotrioctahedron ; the cube is very rare.
Physical and optical properties :
- Hardness : 6.5 to 7.5
- S.G. : 3.61 to 4.5
- R.I. : 1.73 to 1.80 (1.90)
- Crystal System : cubic
Occurrences :
Classical Mineral of alluviums.
Synthetics :
- Y.A.G. : Yttrium Aluminium Garnet (R.I. : 1.83 in view of imitating diamond).
- Y.I.G. : Yttrium Iron Garnet.
- G.G.G. : Gadolinium Gallium Garnet (R.I. : 1.95).
- G.A.G. : Gadolinium Aluminium Garnet.
See also the Latest Gemmological News concerning the garnets.

GARNET from BOHEMIA :
Or Bohemian garnet.
Pyrope Garnet of deep red colour (due to a high chromium content; they are chromium-pyropes).

GARNET from SRI LANKA :
Almandine Garnet.

(BLACK) GARNET :
Name given by the Americans to a Melanite (variety of Garnet andradite).

(BOBROWKA) GARNET :
Name given to a grossular Garnet or more generally to a demantoid Garnet.

(CHROMIFEROUS) GARNET :
Used as synonym of uvarovite.

(CINNAMON) GARNET :
Variety of or synonym of Hessonite of yellow to brown-orange colour.

(HYACINTH) GARNET :
Grossular Garnet of colour orange, red, reddish-brown (old term; now hyacinth means red-brown zircon).

(NOBLE) GARNET :
Synonym of almandine Garnet.

(OLIVINE) GARNET :
Demantoid Garnet of olive green colour.

(ORIENTAL) GARNET :
Synonym of Garnet almandin.

(SIRIAM) GARNET :
Or Syrian Garnet.
Commercial Name for any Garnet of red-violet colour.

(WHITE) GARNET :
Synonym of Leuco-garnet.

GARNETITE :
Synonym of Staurotide.
Or Cipolin (calcareous rock formed almost uniformly of entwined crystals of Calcite) heavily included with garnets.

GARNET JADE :
Variety of grossular Garnet of green colour, also known under the Name of Jade from Transvaal.

GARNIERITE :
Mineral resembling to Steatite (source : Eppler) or to Serpentine (source : Dana), of amorphous appearance.
(Si₂O₅)(Mg.Ni)3(OH)4.
Physical and optical properties :
- Colours : light apple green to emerald green.
- Hardness : 2 to 3
- S.G. : 2.3 to 2.8
- R.I. : around 1.59
- System : triclinic

GAUNGSA :
Burmese Term for designating pale ruby  of mediocre quality and having a average weight of 6 carats.

GAWDONE :
Burmese Term for a star Sapphire.

GEDANITE :
Fossil resin that one designates wrongly with the term " friable amber ".
Physical and optical properties :
- colours : idem as Amber.
- Hardness : 1.5 to 2
- S.G. : 1.06 to 1.07

GISEINGLAS :
German Name designating Obsidian.

GELBMENAKERZ :
German Name designating Sphene.

GELITE :
Synonym of Opal (and sometimes of Chalcedony).

GENEVITE :
Synonym of Idocrase.

GERMAN LAPIS :

Prohibited Appellation ; it is a light grey jasper of Nunkirchen in Germany ; tinted blue to imitate lapis lazuli

GEYSERITE :
Variety of Opal formed near geysers.

GHOSHENITE :
Or Goshenite.
Colourless Beryl.
Stone quite rare.
Physical and optical properties :
- Colours : colourless
For beryl : all colours possible. A name is given to each gem-variety according to its colour : blue = aquamarine, green = emerald, pink = morganite,…
- Transparency : transparent to opaque.
- Hardness : 7.5 to 8
- S.G. : 2.68 to 2.91
- R.I. : 1.564 - 1.602 (Birefr : -0.004 to -0.009)
- Uniaxial negative
- Facies : same as morganite.
- Crystal system : hexagonal
Occurrences :
Product of sodolithic pegmatite.
Brazil, China (associated with scheelite and cassiterite).
Treatments :
Goshenite may adapt a yellow-orangy aspect or a blue colour under gamma-rays.
These colours are not stable to day-light.

G.G.G. :
See Gadolinium Gallium Garnet.

GIBSONITE :
Pink, fibrous variety of Thomsonite from Renfrewshire and from Dumbartonshire in Scotland.

GIGAKU :
Japanese name designating Jade and sometimes the precious stones in general.

GILSONITE :
Synonym of Uintahite.

GIOGETTO :
Italian name for black Coral.

GIRASOL :
This term designates, depending on the place and the authors, different stones, principally they are :
- Term given by the Mexicans to fire Opal.
- Term given to fire Opal.
- Term given to Moonstone.
- Synonym of opalescent Quartz (Milky Quartz with slightly opalescent reflections).
- Term used for a Cat’s eye sapphire presenting a diffuse line, large and indistinct (also called girasol sapphire or oriental girasol).
- Commercial name for glass spheres used in the fabrication of imitation pearls.

(GIRASOL) OPAL :
Synonym of water opal.

(GIRASOL) SAPPHIRE :

Variety of cat’s eye sapphire showing a broad luminous un-sharp line .

GLANCESPAR :
Variety of Andalousite.

GLANZEISENERZ :
German name for Hematite.

GLANZEISENSTEIN :
German name for Hematite.

GLASIGER FELDSPATH :
German name for a Sanidine.

GLASKOPF :
German name for Hematite.

GLASS :

A silicate mixture solidifying without crystallisation ;
- the natural glasses are : obsidian and the glasses of meteoritic origin (tektites ) like moldavite
- the artificial glasses are very numerous ; the two most common kinds are : "Crown glass ", composed of silicon, potassium, soda and calcium ( = common glasses ); " Flint glass " where calcium is replaced by a lead oxide .

(BERYLLIUM) GLASS :

An artificial glass imitating green and blue  stones; containing the element Be .
Hardness : 7
Density : 2.41 to 2.49
R.I. : 1.50 to 1.52

(CRYSTAL) GLASS :

Glass containing at least 13 % of lead oxide .

(FLUOR) GLASS :

Glass containing fluorine, sometimes fraudulesly sold under the name of aventurine .

(LAVA) GLASS :

Synonym for obsidian .

(METEOR) GLASS or (METEORITE) GLASS :

Synonym for green moldavite .

(OPALINE) GLASS :

Artificial white milky glass, sometimes yellowish opaline glass .
- Density : 2.07 and higher
- R.I. : 1.44

(SAPPHIRE) GLASS :

Glass of blue-sapphire colour; Prohibited appellation
One of its varieties, of unknown composition has an exceptional hardness of 6.75 .

(SILICA) GLASS or SILICIUM GLASS:

(1) Natural glass . See Libyan glass .
- colour : pale yellowish green
(2) Artificial  glass made by fusing quartz in oxyhydrogen flame .
- Hardness : 5 (source : Smith) ; 6 (source : Schlossmacher)
- S.G. : 2.2 (source : Smith and Schlossmacher )
- R.I. : 1.44 (source : Schlossmacher ) to 1.46 ( source : Smith )

(THALLIUM) GLASS :

Variety of artificial glass containing thalium
- Density : 5.4

(TOPAZ) GLASS :

Variety of artificial glass imitating topaz
- Density : 4.98
- R.I. : 1.77

(VOLCANIC) GLASS :

Synonym for obsidian

GLASS OPAL :
German name for Hyalite opal or for crystalline salt.

GLASSPATH :
German name for Fluorite.

GLASS SCHORL :
German name for Axinite.

GLAUCOLITE :
Synonym for Sodalite .

GLAUCOSIDERITE :
Synonym of Vivianite.GLESSITE :
A Fossil resin resembling light-brown Amber and not containing any succinic acid .

GLESSUM :
Synonym used for Amber.

GLINKITE :
Variety of Peridot.

GLOW STONE :
English name designating a Chalcedony.

GNAT STONE :
English name for a dendritic Quartz, a moss agate or a Mocha stone with very fine black inclusions
.GOCKUMITE :
Or Gokumite. Variety of Idocrase.

GOETHITE :
FeO.(OH)
As inclusions sometimes present in Quartz in the form of fine needles of yellow, reddish or brownish colour ; or iron oxide forming the yellow points in Sunstone.

GOKUMITE :
Variety of Idocrase.

GOLD BERYLL :
German name for a golden coloured Beryl or synonym of Chrysoberyl (source : Chiser).

GOLD FLUSS :
Commercial name or synonym for aventurine glass.
Glass imitation stone for a sunstone. This imitation resembles but grossly the sunstone and is produced by throwing an amalgam of small copper platelets into a batch of glass.

GOLD OPAL :
Name sometimes given to fire Opal.

GOLD QUARTZ :
White milky Quartz containing small platelets of native gold.

GOLD STONE :
Name given to a glass imitation, imitating Aventurine.

GOLDEN OPAL :
Opal presenting only a golden yellow colour.

GOLDEN SAPPHIRE :
Yellow to yellow greenish sapphire

GOOSEBERRY :
English name for a Grossular Garnet, and more specifically for a variety of a yellow-green colour.

GORGINIE :
French . Term given in ancient times to a very hard Coral.

GOUTTE D’EAU DU BRESIL :
French term. Variety of colourless and limpid Topaz.

GOUTTE de SANG :
French. Name given to a blood-red Spinel.

GOTA de ACEITE :
Spanish term designating the best quality of Emerald.

GOUVERNEURITE :
Synonym of Dravite.

GRABSTEIN :
German Name serving to designate Amber.

GRAMMATIAS :
Name given in ancient times to a sort of Jasper when it had in the middle a white transversal line.

GRANATOÏDE :
Variety of Idocrase.

GRANDIDIERITE :
Collector’s stone
(Mg,Fe₂₊)Al₃[O|BO₄|SiO₄]
Physical and optical properties :
- Colours : bluish green
- Transparency : transparent to translucent
- Lustre : vitreous
- Hardness : 7.5
- S.G. : 2.976
- R.I. : 1.602 - 1.639 (Birefr. : 0.037)
- biaxial negative
- Crystal system : orthorhombic

GRANDITE :
Name designating a garnet of which the chemical composition lies between Grossular and Andradite.

GRANZERITE :
Synonym of Sanidine.

GRAUEISENKIES :
German name for Marcassite.

GRAUPENSCHORL :
Synonym of Tourmaline.

GREASE STONE :
English name for a Steatite.

GREEN GARNET :
here also this name designates several stones :
- English name serving to designate demantoid Garnet and green grossular Garnet is generally called Goose berry garnet.
- Prohibited appellation for designating a green Enstatite.

GREEN-JOHN :
Variety of green Fluorite found in Great-Britain.

GREENLANDITE :
Synonym of Almandine.

GREEN ONYX :
Prohibited Appellation for a chrysoprase or for an artificially green tinted agate

Incorrect term but commercially accepted in the U.S.A., serving to designate a  Chalcedony coloured artificially green.

GREENOVITE :
Prohibited appellation for a Sphene blood red or pink, or manganiferous Sphene of Saint-Marcel.

GREEN QUARTZ :
English Name for Chlorastrolite.

GRENAT TETRAEDALE :
French term. Synonym of Helvine.

GRIQUALANDITE :
Mineralogical name for silicified Crocidolite of yellow colour present as parallel lines in Tiger eye.

GROSSULAIRE : 
French for Grossular.

GROSSULAR :
Or Grossularite.
Ca₃Al₂ (SiO₄)₃
Calcium-aluminium Garnet.
General name for several calcium-aluminium garnets, among them : the Grossular properly speaking, Hessonite, leuco-garnet, hydro-grossular, hyacinth Garnet, Landerite, Romanzowite and Succinite (of Bonvoisin).
Physical and optical properties :
- Colours : colourless, pale yellow, light green, deep green (= Tsavorite), orange (= Hessonite), pink, orangy-red + varieties of massive greens and pinks to red.
- Chromium and vanadium may sometimes replace aluminium, giving a a green tint to the stone. Thanks to the ionic replacement, the stone can appear emerald green (it is than called Tsavorite) or also colourless, which is relatively rare.
- Bi-valent iron may also sometimes replace vanadium and the stone will take in that case a  yellow-orange to brown tint (= Hessonite).
- When the iron and the chromium replace the vanadium, a phenomenon of substitution of oxygen ions by hydroxyde ions is sometimes remarked. The hydrogrossular forms than a stone that is green (confusion with jade), or pink (confusion with thulite).
- Transparency : transparent to opaque
- Lustre : vitreous to waxy
- Fracture : splintery.
- Hardness : 7 to 7.5
- R.I. : 1.73 to 1.75
- S.G. : 3.4 (pink) ; 3.45 to 3.55 (green) => massive grossular;  3.57 to 3.67 => gem grossular.
- Clear spectrum of chrome-vanadium when the colour is intense.
Occurrences :
Mineral of regional metamorphism or of contact metamorphism. Sometimes associated to epidote, to scheelite and to chalcopyrite.
Sri Lanka, Kenya, Tanzania, India, Russia, Bali, Pakistan, California, Mexico.
Inclusions :
The hessonites of Sri Lanka are sometimes included with small zircons.
Under the microscope : image resembling to crumbled paper.
Treatments and imitations:
Doublet using a very pale grossular in view of imitating tsavorite.
Impregnation of plastic material in the hessonites full of fractures in view of dissimulating the fractures in question.

GROTHITE :
Variety of Sphene.

GUARINITE :
Variety of Sphene.

GUARNACINNE GARNET :
Commercial name designating a garnet of yellowish-red colour.

GUNNISONITE :
Variety of impure Fluorite.

GYPSUM :
Decorative and ornamental stone.
CaSO₄.2H₂O 
Physical and optical properties :
- Colours : opaque white to slightly yellowish or pinkish
- Transparency : opaque
- Hardness : 2
- S.G. : 2.2 to 2.4
- R.I. : 1.52 - 1.53 (Birefr. : 0.010)
- Crystal system : monoclinic

GYROLITE :
Variety of Apophyllite.

GYU :
Tibetan name for Turquoise.