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    Nacre the secretion from the mantle of certain molluscs consisting of crystalline carbonate of lime (CaC03) and an organic material called conchiolin. It is the iridescent material of pearl and mother-of-pearl.

    Narwhal ivory an ivory obtained from the "horn" or tusk of the narwhal (Monodon monoceros).

    Nassau pearls conch pearls from West Indies waters.

    Natrolite R.I. 1-48 --- 1-49; S.G. 2-2; H. 5.5; White (fibrous) but other colours are known; Orthorhombic; Norway and Scotland.

    Natural original surface of rough diamond sometimes left in a fashioned stone, often on the girdle.

    Natural glass see Moldavite, Tektite and Obsidian.

    Navette-cut a style of cutting, generally step (trap) cut with a boat-shaped outline, or rectangular with pointed ends. The style is similar to Marquise. See Cuts of stones, Chart 182.

    Needle stone quartz with needle-like inclusions of other minerals, such as rutile, actinolite, etc. Also known as Sagenitic quartz or Rutilated quartz, and Venus or Thetis hair stone.

    Negative crystal a cavity in a crystal having the crystal form of the host mineral. It is one form of inclusion.

    Negative reading the effect shown on a refractometer when the specimen under test has a higher refractive index than the dense glass of the instrument, or higher than the R.I. of the liquid used to make optical contact.

    Neolith an imitation turquoise of German origin.

    Nephrite R.I. 1-60 --- 1-63 to 1-62 --- 1-65; S.G. 2-90 to 3-02; H. 6.5; Monoclinic; Colours, white, leaf-green, emerald-green, dark green; Localities, Canada, China, Siberia, New Zealand, Rhodesia, Turkestan, U.S.A.; Alternative names, Maori stone, New Zealand Greenstone, Axe-stone, and Kidney stone. It is one of the 6`Jades'9.

    Neutron-treated diamonds diamonds which have had their colour altered by neutron bombardment in an atomic pile. Same as "Pile-treated diamonds".

    "Nevada black diamond" a MISNOMER ( COMMERCIAL LABEL ) for obsidian.

    "Nevada diamond" a MISNOMER ( COMMERCIAL LABEL ) for decoloured obsidian.

    "Nevada topaz" a MISNOMER ( COMMERCIAL LABEL ) for smoky obsidian.

    Nevada wonderstone a volcanic rock in stripes of red and buff. Density is about 2 53; Nevada, U.S.A.

    "New jade" a MISNOMER ( COMMERCIAL LABEL ) for bowenite serpentine.

    New Zealand greenstone an alternative name for the nephrite found in New Zealand.

    Niccolite a pale red coloured mineral with a metallic lustre which has a H. of 5 to 51 and a density varying between 7 3 and 7 6. The crystallization is hexagonal but the mineral is always found massive. U.S.A., Canada, and Europe. Also known as kupfernikel.

    Nicol prism a device for the production of polarised light, consisting of a rhomb of Iceland-spar (calcite) cut and cemented diagonally so that the ordinary ray is reflected out of the side of the rhomb and the extraordinary allowed to pass through. Two Nicols are used in a petrological microscope, the first, the polariser, below the stage, and the second, the analyser, in the tube above the objective.

    Nitric acid a powerful mineral acid. One of the constituents of aqua regia.

    Nitrobenzene a liquid, having a value in certain refractive index tests. R.I.= 1 55.

    Non-nucleated cultured pearls Cultured pearls grown by the insertion of a piece of mantle into the mantle of the Japanese freshwater mussel (Hyriopsis schlegeli). These pearls are usually white, small and baroque or bun-shaped. Their nature may be revealed by their strong fluorescence under X-rays and by the peculiar cavities shown by an X-ray picture.

    "Norbide" trade name for an abrasive made of boron carbide.

    Nucleus the central mass around which matter accretes or grows. Generally used with reference to the centre of the pearl.

    Nyf the name given to the skin found on some diamonds from South Africa.