HK Spinel-print - SSEF

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Dr. Michael S. Krzemnicki

Swiss Gemmological Institute SSEFSwitzerland

Spinel:A Gemstone on the Rise

Hong Kong, March 2010

© Swiss Gemmological Institute SSEF

Spinelof 110 ct

fromTajikistan

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What is Spinel?


Spinel as a gemstone is mainlymagnesium-aluminium-oxide

MgAl2O4

More general: the spinel group

(Me2+) (Me3+)2 O4

Me2+ tetrahedral coordinationMe3+ octahedral coordination

Cubic

Colours of Spinel


Pure MgAl2O4 is colourless.

Trace elements such as chromium, vanadium, iron, cobalt produce manydifferent shades of colours such as red, pink, orange, purple, blue, black...

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Crystal Shape of Spinel


Most spinel crystals are octahedral,but also other cubic forms exist suchas rhombic dodecahedra...



Spinel octahedron

But remind:Rubies may also have a pseudo-octahedral “shape”

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Spinel octahedron

Spinel crystals are often twinned.

Formation of spinel


Many gemquality spinels are formed in marbles,similar to rubies.

- Spinel in dolomite marble (Mg-Ca-carbonate)- Ruby in calcite marble (Ca-cabonate)

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Formation of spinel


Often occurring together with magnesium minerals such as clinohumite andchondrodite (Mg-Fe-silicates), phlogopite (K-Mg mica), pargasite (Na-Ca-Mg-Fe amphibole).

This 6.76 ct clinohumite is a fine exampleof the "sunflower stone" from Kuh-i-Lal.

© Wimon Manorotkul / Pala International

Chondrodite © H.A. Hänni. SSEF

Pargasite © C.Hauzenberger

History of spinel


In ancient times, spinel was not known as amineral and many of the large historic „balassrubies“ from Central Asia and Persia werelater identified mineralogically as spinels.

One of the finest examples of a Badakhshanspinel is the Black Prince “ruby“ (~ 140 ct)which is mounted in the front of the ImperialState Crown of Great Britain.

These large and often just slightly polishedand engraved spinels went through thehands of the kings and shas of central Asia,Persia and India and were only described bya few adventurous European travellers suchas Jean-Baptiste Tavernier and others at thattime.

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Exceptional historic spinel necklacecertified at SSEF


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Properties of Spinel


Spinel is cubic,

RI for natural gem quality spinelsusually approx. 1.718

SG 3.60

Lustre: High vitreous

Hardness: 8 (Mohs scale)

Clarity: Often clean stones, no cleavage

Colour: by trace elementsin many shades from red, pink, orange, purple, to blue and blackno pleochroism,usually no colour zoningred spinels often with Cr-luminescence, resulting in bright red colour



Most spinel crystals are octahedral,but also other cubic forms exist suchas rhombic dodecahedra...

Octahedral shape perfect fitting for cutting proportions

Faceted red spinel (2.24 ct) with spinel crystal(15 ct), both from Burma.© Photo: Mia Dixon, Palagems International

© M.S. Krzemnicki, SSEF

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Spinels are generally rather pure gemstones without distinct colour zones, incontrast to rubies, which are often quite included and fractured and colour zoned !

Pure spinel from Pamir (Tajikistan) of approx. 40 cts. Rubies from Vatomandry, Madagascar(above) and Mong Hsu, Burma (below)

Spinel is usually not treated!


Heating experiments on spinels show no or only limited enhancement, apartfrom clarifying some turbidity in spinel.Heated spinels are detected by microscopy and Raman-microspectrometry(Photoluminescence) at SSEF.

A 1.17 ct spinel from Tanzania before (left) and after heatingfor 5 hrs at 1400 °C, in oxidizing conditions (right).© S. Saeseaw et al., GIA (2009)

Ruby heated and withglassy residues inhealed fissures.© H.A. Hänni, SSEF

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Synthetic spinel

Most are produced bya flame-fusion process (Verneuil).

Visit at Djevahirdian SA,Monthey, Switzerland


Verneuil synthetic spinel


Large quantities are produced for technical purposes(e.g. scratch resistant glasses for watches).

Usually these synthetic spinels are highly pure, andoften show a colour range from blue - light blue,green, and colourless.

Verneuil synthetic spinel.Experiences gemmologists generally can separatethem easily from natural spinels based onmicroscopy, UV fluorescent reactions and slightlyincreased values for RI (1.725 instead of 1.718)and SG ( 3.65 instead of 3.60).

The difference in these physical properties is dueto a non-stochiometric chemical composition ofthese Verneuil synthetic spinels, i.e. they containtoo much aluminium. Airbubbles in a Verneuil synthetic spinel.

As a consequence, there crystal structure is slightly deformed, resulting in strain doublerefraction under the polariscope and in broadening of the peaks in the Raman spectrum.

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Flux-melt synthetic Spinel


Flux residue in synthetic spinel

Flux-melt synthetic spinels are known since the 90ies,but quite rare. As a consequence of the increasedinterest and value of red and intense blue spinels, theSSEF has tested recently a number of such syntheticspinels from the market.

Flux-melt synthetic spinelform as nice octahedralcrystals, mostly in saturatedred and blue colours. Thesestones are not easy toseparate from natural spinel.


Raman spectra offlux-melt synthetic spinel

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Luminescencespectra of

flux-melt syntheticspinel


Sources of spinel


As for ruby and sapphire, spinel of fine quality is oftencertified. The clients often require an origin determinationof the spinel, especially for stones from Burma, Tajikistan(Pamir), Vietnam and Tanzania.

At SSEF, origin determination is basedon microscopy (inclusions), traceelement distributions and UV-Vis andRaman spectroscopy.The origin of spinels can not bedetermined in all cases due tooverlapping properties of spinels fromdifferent sources.

LA-ICP-MS, for analysis of trace elementsand isotopes.

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Major sources of spinel


Madagascar

Tajikistan

Siam (Thailand)

Tanzania

Burma (Myanmar)

Ceylon (Sri Lanka)

VietnamAfghanistan

Ceylon (Sri Lanka)

© www.mygeo.info

Tajikistan (Pamir)


Tajikistan is a classical source for large spinels since ancienttimes. they usually show a pinkish red to pink colour.

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Tajikistan (Pamir)


Pierre Lefèvre, gemmologist at SSEF with asuperb spinel of 110 cts from Tajikistan.

Burma (Myanmar)


Spinel from Burma is highly appreciated in the market especially forits outstanding saturated red colour.

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Burma (Myanmar)


But Burma is also the source of spinels in many pastel colours.

Madagascar


Colour changing spinel (84 ct) from Madagascar

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Vietnam


Apart from red and pink spinels,Vietnam has recently produced finespinels in light purple, light blue andsaturated cobalt blue.

Vietnam


Characteristic crossed inclusions in spinelfrom Vietnam.

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Tanzania


Main sources for spinel in Tanzaniaare Matombo, Mahenge and

Tunduru in the south.

Matombo

MahengeTunduru

Tanzania, Mahenge


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Tanzania, Mahenge


Oriented planes of fine „dust“ precipitates in spinel from Tanzania

© H.A. Hänni, SSEF

Tanzania, Mahenge


Matombo

Mahenge

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Tanzania, Mahenge


Tanzania, Mahenge


Geological map of the Mahengefold nappe. Indicated in blue arethe marble rocks. The spinels (andrubies) are found at different spotsin these marbles or near-by eluvialand aluvial gravels.

Spinel from Mahenge (3.4 ct), showing acharacteristic attractive bright red colour.

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Tanzania, Mahenge


In 2007, a 52 kg (260‘000cts) spinel was discoveredin Mahenge. Although onlypartly of gem quality, thisresulted in a sudden supplyof large (up to 50 cts) andmany small vibrant pinkishred spinels (totally severalthousand carats) in themarket.

© W. Spaltenstein and D. Weinberg, Multicolour, Thailand

Trip to Mahenge, October 2009


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Tanzania, Mahenge


Tanzania, Mahenge


MahengeMorogoro Province

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Going to the mine:Epangko, south of Mahenge


Epangko, Spinel mines


Gemmologist Walter Balmerat the Epangko spinel mines.

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Epangko, Spinelmines


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Thank you for your attention...

High Noon in Mahenge...

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HK Spinel-print - SSEF