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    Labradorescence name given to the play of colour seen in the labradorite feldspar.

    Labradorite a variety of plagioclase feldspar showing a play of colour (labradorescence). See Feldspar.

    Labrador moonstone labradorite.

    Labrador spar labradorite.

    "Lake George diamond a MISNOMER ( COMMERCIAL LABEL ) for rock crystal.

    Lake Superior greenstone chlorastrolite.

    Lamellar structure when the mineral is composed of thin layers of scales like the leaves of a book the structure is said to be lamellar.

    Landerite see Xalostocite.

    Lap the name applied to the disc upon which gemstones are cut and polished. For diamond the lap is of hard cast iron. It is also known as scaife. With the softer gemstones the grinding laps are made of copper, gunmetal or lead, while pewter or wooden laps, sometimes faced with cloth or leather are used for polishing.

    Lapidary the name applied to craftsmen who cut and polish gemstones other than diamonds. The craftsmen who fashion diamonds go under the name of "Diamond cutters" and not lapidaries.

    Lapis, False see False lapis.

    Lapis-lazuli (Lazurite) a mixture of several minerals, chief of which are lazurite, and calcite; R.I. 1-50; S.G. 2-75 to 2-90; H. 5.5; Colour, deep blue often with spangles of pyrites; Localities, Afghanistan, Chile, Siberia and California.

    Lapper the name applied to the diamond cutter who cuts the main facets of a brilliant cut stone.

    Lasque a flat thin diamond used by Indian jewellers. May be the origin of the term "laxy diamond" used in the trade for rather flat diamonds cut in the brilliant style.

    Lauegram the characteristic figure of spots produced on a photographic film when a narrow beam of X-rays is passed through a single crystal, due to diffraction from the atomic layers. The six-spot and four-spot X-ray photographs of pearls and cultured pearls are a modification of this effect.

    "Lavernite" a name applied to the synthetically produced periclase.

    "Laxy diamonds" Slang trade term for rather flat brilliant cut diamonds. See Lasque.

    Lazulite R.I. 163; S.G. 31; H. 5 to 6; Monoclinic; Colour, blue; Localities, Austria and U.S.A.

    Lazurite see Lapis-lazuli.

    Lead glass flint glass containing a high percentage of lead oxide. This increases the dispersion but decreases the hardness. See Flint glass.

    Lechosos opal see Opal.

    Lepidolite R.I. mean 1-55; S.G. 2-8 to 2-9; H. 3.5; Monoclinic; A rose red ornamental stone from Russia, Madagascar and U.S.A.

    Leucite KAl(Si3O2)2; R.I. 1 51; S.G. 2 48; H. 51 to 6; Cubic; Colourless; from Italy.

    Leuco-sapphire colourless corundum, which see.

    Leveridge gauge A stone gauge on the principle of calipers with the measurements shown on an indicating dial.

    Light 1 a form of radiant energy, travelling through space at the speed of 186,285 miles per second, which gives rise to the sensation of sight. It is an "octave" of the electromagnetic spectrum and differs from wireless, heat, ultraviolet and X-rays solely in wave-length.
    2 Interference of; see Interference of light.
    3 Monochromatic; see Monochromatic light.
    4 Reflection of; see Reflection of light.
    5 Refraction of; see Refraction of light.
    6 Speed of; see Speed of light.

    Lime name commonly applied to calcium oxide, also used for a polishing product consisting of unhydrated Dolomite containing a high percentage of magnesia. It may be said to be a double oxide of calcium and magnesium and has almost entirely replaced crocus.

    Limonite a hydrated iron oxide which is often the matrix in which is found turquoise. Brown in colour, limonite is sometimes cut with turquoise to produce what is known as turquoise matrix.

    Linde emerald a synthetic emerald made by the hydrothermal method by the American firm of Linde.

    "Linobate" lithium niobate, a synthetically produced crystal used as a gemstone.

    Lintonite a plain green variety of thomsonite.

    "Lithia amethyst (Lithium Amethyst)" a MISNOMER ( COMMERCIAL LABEL ) for the kunzite variety of Spodumene, which see.

    "Lithia emerald" a MISNOMER ( COMMERCIAL LABEL ) for the green hiddenite variety of spodumene, which see.

    Lithium niobate (LiNbO31; a synthetically produced crystal used as a gemstone under the name 'linobate'. The hardness is 6; the density 4 64 and the refractive indices 2 21-2 30.

    Liver opal see Opal.

    Logarithmic tables Calculation tables.

    Loupe jewelers see Lens.

    Low-type zircon the division of zircon consisting of those stones, generally green in colour, which have low physical and optical constants and are practically amorphous. This type, which is said to be the result of a more or less complete breakdown of the crystalline types, shows practically no birefringence.

    Lozenge the name applied to a trap-cut stone having a lozenge-shaped outline.

    Lucidoscope the name applied to an early method of cultured pearl testing. The pearl is immersed in monobromonaphthalene, illuminated by a strong source of light and viewed microscopically. On being slowly turned a natural pearl will show an equal density to the light, whereas a cultured pearl may show a banded structure when the mother of pearl layers of the bead nucleus are in the same plane as the light rays.

    Lucinite variscite from Lucin, Utah.

    Lumachella or Lumachelle "Fire Marble", a marble containing fossil shells which show a play of colour reminiscent of opal.

    Luminescence the effect noticed in some substances of giving out visible light when they are rubbed or scratched (Triboluminescence) or when they are irradiated with invisible electro-magnetic radiations (Fluorescence, Phosphorescence and Thermoluminescence).

    Lunette the name applied to a trap-cut stone with an outline having one side a straight line and the other a segment of a circle. See Cuts of stones, Chart 182.

    Lustre the brilliancy of a stone by reflected light, determined by the amount of incident liaht reflected from its surface. Types of lustre as follows:
    (1) Adamantine; as in diamond and some zircons. Possessed only by minerals of high refractive index.
    (2) Vitreous; glass-like as in quartz and most gemstones.
    (3) Resinous; as in certain garnets.
    (4) Waxy; as in turquoise.
    (5) Pearly; as in moonstone.
    (6) Silky; as in satin-spar.
    (7) Metallic; as in gold.
    The lustre of minerals may be of different degrees or intensity and are then described as under: Splendent; the surface reflects light as in a mirror Shining; objects are reflected indistinctly. Glistening or Glimmering; denotes a still more feeble lustre. Dull; little or no lustre.

    "Lux sapphire" a MISNOMER ( COMMERCIAL LABEL ) for Iolite.

    "Lynx sapphire" a MISNOMER ( COMMERCIAL LABEL ) for the pale blue shade of Iolite, which see.