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    Obsidian a volcanic glass; R.I. 1-50; S.G. 2-3 to 2-5; H. 5.5; Amorphous; Colour, black, red and brown. See also Moldavite.

    Obus the name applied to a trap-cut stone oblong in outline but with one end brought to a point. See Cuts of stone, Chart 182.

    "Occidental topaz" a MISNOMER ( COMMERCIAL LABEL ) for yellow quartz.

    Octahedrite alternative name for anatase.

    Octahedron a crystal form having eight faces. It may be described as being two pyramids, each formed of four equilateral triangles, placed base to base; a crystal form of the cubic system.

    Odontolite or "Bone turquoise" fossil bone or ivory naturally coloured blue by phosphate of iron. S.G. 3 0 to 3 25; H. 5; Organic structure shown under lens and effervesces with hydrochloric acid. Colour, blue.

    Off-colour the term applied to diamonds having a yellowish or brownish tinge.

    Oil pearls "pearls" cut from the sea snail shell, also called Antilles pearls.

    Oligoclase see Feldspar.

    Olive oil an oil having a value in certain refractive index tests. R.I. = 1-47.

    Olivine the mineralogical name for the iron magnesium silicate, the gem variety of which is Peridot. Olivine, often misspelt olivene, is a MISNOMER ( COMMERCIAL LABEL ) for demantoid garnet.

    Once the weight the term used in the "base" system of pearl price calculation for 1/- base.

    Ontario moonstone peristerite variety of feldspar from Ontario.

    Onyx see Chalcedony.

    Onyx Marble name for banded calcite.

    Opal a silica gel; R.I.1-44 to 1-47; S.G. 1-95 to 2-20; H. 5 to 6.5; Varieties:

    Precious opal showing good play of colour White on white ground ---- Black on black ground.
    (1) Harlequin opal has patches of colour of a regular size.
    (2) Fire-opal; semi-transparent of orange to red colour.
    (3) Opal matrix is opal cut showing some of the ironstone matrix.
    (4) Girasol transparent blue-white with a red play of colour.
    (5) Lechosos opal; a variety showing a deep green play of colour. Prase opal is coloured green.
    (6) Memlite or Liver opal is grey or brown.
    (7) Milk opal is yellowish, bluish-white, or white in colour.
    (8) Mexican Water opal; a clear colourless or yellowish opal showing a play of colour.
    (9) Moss opal; opal with dendritic inclusions.
    (10) Resin opal; yellow in colour with a resinous lustre.
    (11) Wood opal; an opal pseudomorph after wood.
    (12) Hyalite is a colourless, glass-like opal.
    (13) Hydrophane is a dehydrated opal which becomes opalescent when placed in water.
    (14) Cacholong is a white porcelain-looking type.

    Localities, (Precious Opal) Hungary, Australia; (Fire Opal) Mexico.

    Opal doublet a composite stone consisting of a thin slice of opal backed with a piece of "potch opal", or with a piece of black onyx or special black glass called "opalite".

    Opal, treated opal which has been stained with a black carbonaceous compound to give a black colour to an otherwise white opal.

    Opalescence a reflection of a milky or pearly light from the interior of a mineral. Also used by some as an alternative to Iridnescence.

    Opalite a name applied to some types of common opal.

    Operculum the name applied to the calcareous disc found at the head of certain univalve molluscs and used by the animal as a door to close the aperture of the shell. The opercula from the shellfishes Turbo petholatus, found in the South Seas north of Australia, have a limited application in jewelry.

    Ophicalcite the mineral name for serpentinous marble, such as Connemara marble.

    Optical sign in addition to the classification of minerals into optically "uniaxial" and "biaxial" (q.v.), a further subdivision is made into those which are optically "positive" and those which are optically "negative". The conventions are as follows: (a) with uniaxial minerals, those in which the extraordinary refractive index is greater than the ordinary are said to be positive, while those in which the extraordinary index is the lower are termed negative. (b) in biaxial stones, those in which the intermediate index, 6, is nearer to the lowest index, a, than to the greatest index, y, are termed positive, while those in which ,B is nearer y are termed negative. Examples: tourmaline is uniaxial negative; topaz is biaxial positive.

    Optic axes directions of single refraction in doubly refracting stones. In the tetragonal and hexagonal systems there is one such direction and such crystals are termed uniaxial; rhombic, monoclinic and triclinic crystals have two directions of single refraction and are termed biaxial.

    Optic axial angle the acute angle subtended by the optic axes in biaxial crystals, usually denoted as 2V, or 2E as seen in air.

    Orbicular the term used for a mineral or rock containing numerous spherules solidly encased, e.g., orbicular jasper.

    Ordinary ray a ray in a doubly refracting stone which behaves similarly to a ray passing through isotropic material, in that it travels with the same velocity whatever its direction in the stone. This ray is only possible in crystals of the tetragonal, hexagonal and trigonal systems (uniaxial crystals).

    Organic produced by vital processes. In chemistry the compounds of carbon are termed "organic" compounds.

    "Oriental" (Emerald, Amethyst, Topaz, etc.) a prefix sometimes used to describe corundum, having similar colour to the stone described in the second part of the name viz.: "Oriental Amethyst" is violet sapphire. A most undesirable MISNOMER ( COMMERCIAL LABEL ).

    "Oriental Alabaster" see Egyptian alabaster.

    "Oriental emerald" this term, besides being used as an incorrect name for the green corundum, has also been applied to the green, chlorospinel type of natural spinel. Whether used for corundum or spinel the term is definitely a MISNOMER ( COMMERCIAL LABEL ).

    Orient of pearl the iridescent surface sheen of gem pearl. It is due to the combined effect of a play of colour due to interference of light at thin films, the thin plates of the nacreous layer; and to diffraction from the fine edges of the plates.

    Ornamental stones a term used for those minerals, which, through lack of transparency, own their beauty solely to their body colour and are used mainly for small carvings and objets d'art. Such stones are Malachite, Lapis lazuli, marble, etc.

    Orthoclase monoclinic feldspar, which see.

    Orthorhombic a crystal system for which the term Rhombic is an alternative.

    Orthotoluidine a liquid having a value in certain refractive index tests. R.I.= 1 57.

    Osmium S.G. 22 5; a metal of the platinum group having the distinction of being the heaviest known metal and a very high melting point (2,700 C. about). Osmium has no annlication in jewelry.

    Oxidation the chemical change by which oxygen is added to an element or compound.

    Oxides compounds of oxygen with another element. Examples are Coruridum and Quartz.