Diaspore

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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
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 aquired 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.

Contents

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; foilated.

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 colour 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.

References

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