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"Fabulite" trade name for strontium titanate.


Faces, Crystal see Crystalfaces.


Facets the name applied to the plane geometrical faces of a polished stone. The name is sometimes used also for the inclined faces at the top of a prismatic crystal.


Falcon' - eye" see Hawk's-eye.


False cleavage see Parting.


False lapis name sometimes applied to the stained chalcedony made to imitate lapis lazuli; also known as "Swiss lapis" or "German lapis".


"False topaz" see Quartz (citrine).


Fancy diamonds coloured diamonds which have a definite shade of colour, such as: canary yellow, green, blue, red, pink, brown, etc. Green and yellow coloured diamonds may have been treated. See also Coloured diamonds.


Fancy shapes the name used for oblong, pentagon, half-moon and other fancy shapes in cut-stones. See Chart 182.


Fashioning of stones A term used for the cutting and polishing of stones.


Fayalite one end member of the isomorphous series of which peridot is a variety. See Peridot.


Feathers layers of liquid or crystalline inclusions having a feather-like appearance sometimes found in natural crystals of rubies and sapphires, etc.


Feldspar (Felspar) name given to an important group of rockforming minerals, consisting of a silicate of aluminium with either potassium (Orthoclase and Microcline) or sodium and calcium (Plagioclase).
(1) Orthoclase; R.I. 1-52 --- 1-53 to 1-53 --- 1-54; S.G. 2-55 to 2-58; H. 6 to 6.5; Monoclinic; Colours yellow and colourless with opalescence (Moonstone); Localities, yellow, Madagascar; Moonstone, Ceylon, Switzerland and Burma.
(2) Microcline; R.I. 1-52 --- 1-53; S.G. 2-54 to 2-57; H. 6 to 6.5; Triclinic; Colour, bright verdigris green (Amazon-stone); Locality, Pikes Peak, Colorado (U.S.A.). The plagioclase group forms an isomorphous series from Albite to Anorthite. Only two members of this group come within the purview of gemmology, viz. Oligoclase (Sunstone or Aventurine) and Labradorite.
(3) Oligoclase (Sunstone or Aventurine); (70% to 90% of albite and 10% to 30% fanorthite);R.I. 1-5--- 1-55; S.G.262 to 2 65; H. 6 to 6.5; Triclinic; Colour, spangled bronze due to included flakes of haematite or goethite; Locality, Norway, Canada, etc.
(4) Labradorite; (30 % to 50 % of albite to 50 % to 70 % of anorthite); R.I. 1-56 --- 1-57; S.G. 2-70 to 2-72; H. 6 to 6.5; Triclinic; Colour, blue and grey with play of colour; Locality, Labrador, Canada.


F.G.A. (Fellow of the Gemmological Association) the suffix used by holders of the fellowship diploma of the Gemmological Association of Great Britain.


F.G.A.A. the suffix used by holders of the fellowship diploma of the Gemmological Association of Australia.


Fibrolite R.I. 1 658-1 677; S.G. 3 25; H. 7.5 Rhombic; Colours, pale blue and greenish; Localities, Burma and Ceylon. Alternative name "Sillimanite".


Figure stone a term used for material used for small stone carvings. It is generally agalmatolite. Fine light brown; a classification of gem diamond. Fine silver cape; a classification of gem diamond.


Fire the flashes of spectrum colours seen from the crown of a stone and due to dispersion, which see.


Fire Agate a layered dark brown color thats produces many and varried brilliant colors as light strikes the surface if cut or polished prorerly.


Fire-marble see Lumachella.


Fire marks small cracks seen along the facet edges in corundum. They are due to overheating on the polishing lap. They are seen both in natural corundums and in synthetic corundums, but are usually more prevalent in the latter.


Fire-opal see Opal.


"Fire pearl" a MISNOMER ( COMMERCIAL LABEL ) used for billitonite.


Firestones the name applied to rock crystal which has had cracks artificially produced by heat. See Quartz (iris quartz).


First bye a classification of gem diamond.


First cape a classification of gem diamond.


First water term used for diamonds which are perfectly limpid and free from flaws.


Flame-fusion process a process used in growing synthetic single crystals. A high temperature flame being used. The Verneuil process is a flame-fusion method.


Flats term used for those diamond crystals which are distorted octahedra.


Flaws may be fractures or cleavages either on the surface or running partly or completely through the stone. See Feathers.


Fleches d'amour see Quartz, Venus' Hair Stone.


Flint glass glass containing lead oxide which gives high dispersion. The material used for imitation gem stones usually has a range of R.I. from 1-58 to 1-68, and S.G. 3-15 to 4-15. The hardness is low in this type.


Fluorescence the effect exhibited by certain materials of producing visible light on being irradiated with invisible ultraviolet rays, cathode rays, X-rays or radium emanations. If the effect is continued after the removal of the exciting radiation it is then termed phosphorescence. See also Luminescence, and Chart 165.


Fluorite mineralogical name for fluorspar, which see.


Fluorspar R.I. 1-43; S.G. 3-17 to 3-19; H. 4; Cubic; Colours, violet, green, yellow, orange, blue, red, pink, brown and colourless; Locality, England, etc. Massive variety, Blue John (Derbyshire spar), Castleton, Derbyshire(England).


Flux fusion (Fluxed melt) a method of growing crystals,similar in many respects to crystallization from aqueous solutions, but the solvents used are solids with relatively high melting points. Some synthetic emeralds and rubies are grown by this method.


Foiled stones gemstones set with silver or coloured foil at the back in order to lighten a dark stone or to enhance the color 5 of a pale stone.


Form-birefringence the, usually small, birefringence commonly seen in material having a mineral of one refractive index embedded in another which has another index. Often seen in chalcedony.


Forsterite one end member of the isomorphous series of which peridot is a variety. See Peridot.


Fossil ivory ivory obtained from the tusks of extinct mammoths whose bodies have been preserved in solid ice since Ice Age times. Found mainly in Siberia and North America. Some of this material is suitable for use in the arts. See Mammoth ivory.


Fossil resin see Amber.


"Fossil turquoise" a MISNOMER ( COMMERCIAL LABEL ) for odontolite. See Odontolite.


Four-point see Diamond point.


Fracture the term used to describe the surface of a stone when chipped or broken. The character of the surface varies with different stones and names are applied to the different kinds: Conchoidal; when the surface takes the form of many more or less concentric ridges resembling the lines on certain shells. Even; when the surfaces are smooth and even. Uneven; when the surfaces are rougher, this is sometimesternied "hackly fracture". Splintery; when the substance breaks into long splinters. Fibrous substances produce this type.


Fraunhofer lines dark lines seen in the spectrum of the sun, which represent the absorption of certain wave-lengths of light by elements present in the outer-chromosphere. Certain of these lines are denoted by alphabetical letters and may be used for calibration. See Chart 155.


Freshwater pearls pearls found in the pearl mussels which inhabit inland waterways of Europe and America. See Mussel pearls.


Frictional electricity charge of electricity developed by certain substances vigorously rubbed with a cloth; diamond, tourmaline and topaz taking on a positive charge and amber a negative one. (Some plastic imitation embers also show this effect.)


Friedelite R.I. 1 63-1 66; S.G. 3 07; H. 4 to 5; Trigonal; Rose red to Orange-red; Localities, France, Sweden and the U.S.A.


Frosted Diamonds diamond crystals which have a frosted appearance but are not so heavily coated as "coated" crystals. See Coated diamonds.


Fusibility the relative fusibility of minerals, generally determined from a scale of six minerals, suggested by Von Kobell, in which the temperature of fusion is understood to increase by approximately equal amounts. Von Kobell's scale is as follows:


1. Stibnit.................................... melting at 525 C.
2. Natrolite ................................................. 965 C
3. Almandine garnet.................melting at 1,200 C
4. Actinolite...............................................1,296 C
5. Orthoclase..............................................1,300 C
6. Bronzite.................................................1,380 C