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    Habit the characteristic crystal form displayed by a mineral. Habit may vary with locality.

    Haematite R.I. 2-9 --- 3-22; S.G. 4-9 to 5-3; H. 5.5 to 6.5; Trigonal; Colour, black; Localities, England, Germany, Scandinavia and U.S.A. Sometimes used to imitate the black pearl. The name hematite should not be used for the heliotrope variety of chalcedony.

    Half-facets see break facets.

    Hambergite R.I. 1-55--- 1-62; S.G. 2 to 3-5; Rhombic; Colourless; Locality, Madagascar.

    Hardness the relative ability of one type of stone to wear on another type of stone. Scale of 10 as hardest to 1 as softest.

    Hardness (H) points or pencils small conical fragments of minerals set in a pencil-shaped holder for ease in testing the hardness of a stone.
    Diamond (10)
    Sapphire (9)
    Topaz (8)
    Quartz (7)
    Feldspar (6)

    Harlequin opal; see Opal.

    Hauynite a blue minera1 of cubic symmetry, it is a sodium aluminium silicate and is one of the constituents of Lapislazuli; also known as Hanyne.

    Hawk's-eye quartz pseudomorphous after crocidolite (blue asbestos) which has not suffered oxidation. See also Crocidolite.

    Heat treatment the controlled heating of certain stones, such as zircon and topaz, in order to effect a change of colour. See Chart 178.

    Heavy liquids liquids having a high density and suitable for the determination of the specific gravity of gem-stones. See Chart 118.

    Heliodor yellow beryl.

    Heliotrope see Chalcedony.

    Hematite see Ha'matite.

    Hemihedral forms those crystals which show only half the number of faces required to conform to the symmetry of the normal crystal.

    Hemimorphism the peculiarity possessed by certain crystals, presenting different forms at opposite ends of an axis of symmetry.

    Herderite R.I. 1 59-1 62; S.G. 3 0; H. 5; Greyish; Monoclinic; Saxony and U.S.A.

    Hessonite see Garnet, Grossular.

    Hexagonal one of the crystal systems. see Chart 111.

    Hexagonite see Tremolite.

    Hiddenite see Spodumene.

    Hippopotamus ivory an ivory obtained from the canine teeth of the hippopotamus. (Hippopotamus amphibus).

    Holohedral forms those crystals which show the full number of faces for the full symmetry of the system.

    Homogeneous the term used for material which is composed of similar parts of elements. It is uniform throughout.

    "Honan jade" an Oriental name variously applied to a jade and quartz mixture, or to serpentine or soapstone.

    "Hope sapphire" a MISNOMER ( COMMERCIAL LABEL ) that was suggested for the synthetic blue spinels that were the outcome of experiments in the production of synthetic blue corundum. The use of magnesia as a flux to cause even distribution of the blue colour gave the unexpected result that spinel was produced and not corundum.

    Hornblende a rock-forming mineral similar in composition to nephrite, may be black or green (Smaragdite).

    "Hot Springs diamond" a MISNOMER ( COMMERCIAL LABEL ) for rock crystal.

    Howlite an opaque white ornamental stone veined with black consisting of an aggregate of monoclinic (?) crystals; R.I. 1-59; S.G. 2-58; H. 3.5; has been stained a turquoise-blue colour; California.

    "Hungarian cat'seyes" an unsatisfactory name for quartz cat's-eyes from Bavaria.

    Hungarian opal the name applied to opal from the Cervenica mines, Hungary.

    Hyacinth name applied to the orange-red zircon or to the similarly coloured hessonite garnet. An obsolete name.

    Hyalite see Opal.

    Hydrochloric Acid a powerful mineral acid formed by the solution of the gas HCI in water. One of the constituents of aqua regia.

    Hydrogrossular correct name for the massive grossular garnet, which see.

    Hydrometer an instrument for determining the density of liquids.

    Hydrophane see Opal.

    Hydrostatic weighing the direct weighing method for determining specific gravity. See Chart 117.

    Hydrothermal method a method of growing crystals by the use of a saturated water solution at a pressure of 1,000 atmospheres and temperatures of less than 400 C. An autoclave is used in this process.

    Hypersthene R.I. 1-67-1-68 to 1-69 --- 1-70; S.G. 3-3 to 3-4; H. 5 to 6; Rhombic; Colour, dark with metalloidal iridescence. Hypersthene is related to enstatite an intermediate form being bronzite.