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16.32: Diaspore

  • Page ID
    4716
  •  

    Diaspore
    Chemical composition AlO(OH)
    Crystal system Orthorhombic
    Habit Tabular, sometimes fibrous
    Cleavage Perfect to imperfect
    Fracture Conchoidal
    Luster Bright vitreous to pearly (on cleavage surface).
    Hardness 6.5 - 7
    Optic nature Biaxial +
    Refractive index 1.682 - 1.752
    Birefringence 0.040 - 0.048
    Specific gravity 3.2 - 3.5
    Lustre Vitreous
    Pleochroism Moderate

    Figure\(\PageIndex{1}\): Color-changing diaspore from green in daylight to brown in incandescent light - also known as “Zultanite” in the gem market.
    Photo courtesy of Chaman Golecha, Gem Testing Laboratory, Jaipur.


    Diaspore as a plausible gemstone has been reported first around 1977 when a large enough crystal was examined at the Gem Testing Laboratory in London. This, usually colorless, material showed good color-change from pale bluish-green to pale pinkish-brown. Turkey was the origin of this color-change material. When Zultanite Gems LLC acquired the mining location, this color-change variety of diaspore was marketed under the tradename "zultanite®". At present (2007) sole distribution rights of "zultanite®" are granted to GemsTV.

     

    Chemical composition

    AlOOH + Mn (or Fe) as impurity (a dimorph of boehmite).
    Mn or Fe may substitute Al in the lattice.

     

    Crystallography

    Orthorhombic.
    Habit: crystals are elongated plates (tabular); acicular needles also massive; foliated.

     

    Diagnostics

    Diaspore may be confused with peridot due to the strong doubling of back facets, but diaspore is usually much paler in color. Iron-rich green sapphire has an absorption spectrum close to diaspore.

     

    Color

    Colorless, pale bluish-green to pale green, pale pinkish-brown to pale brown.

     

    Diaphaneity

    Transparent to opaque.

     

    Specific Gravity

    S.G.:3.2 - 3.5 (mean = 3.4)

     

    Cleavage

    Perfect in one direction {010} and imperfect on {110} and {210}.

     

    Refractometer

    Optic nature: Biaxial positive.
    Refractive index range: nα = 1.682 - 1.702, nΒ = 1.705 - 1.725 and nγ = 1.730 - 1.752. Mean = 1.702 - 1.750.
    Maximum birefringence: 0.040 - 0.048.

     

    Spectrum

    Not diagnostic but Turkish stones 471, 463, 454 (much similar to iron-rich sapphire).

     

    Luminescence

    UV fluorescence: May show dull pale yellow (LW) and green (SW)

     

    Pleochroism

    Strong depending on body color and source. Blue / Pale green and rose to dark red.

     

    Magnification

    Diaspore is a type II stone on the GIA clarity scale, hence small inclusions are usually found.

     

    Phenomena

    • Color-change
    • Cat's-eyes

     

    Simulants

    Sapphire No strong doubling of facets; higher heft; higher R.I., unixial figure.
    Sillimanite Lower S.G., lower RI and lower birefringence.
    Peridot Lower RI and slightly lower birefringence.

     

    Occurrence

    Geological occurrence:
    In metamorphosed limestones, chloritic schists and altered igneous rocks. Also in bauxite deposits.

    Geographical locations:
    Memaris, Turkey (gemmy pale brown); Chester, Massachusetts (with corundum); Pennysylvania (fine transparent colourless to brown); Hungary; South Africa in Postmasburg district - Manganiferous variety; Cornwall, England; Greenland; Norway; Sweden; France; Switzerland; Germany; Greece; USSR; Japan; China; Turkey.