Gemstones The Fascination of Colour€¦ · 1! Presentation by Dr. Michael S. Krzemnicki Swiss...

1!

Presentation by

Dr. Michael S. Krzemnicki

Swiss Gemmological Institute SSEF

Photos © M.S. Krzemnicki, SSEF, except where indicated otherwise

Gemstones &

The Fascination of Colour

Christie’s Spring Auctions 2010 Lecture

Sunday 30th May 2010

© Swiss Gemmological Institute SSEF!

Colour...!

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© Swiss Gemmological Institute SSEF!

...is emotion!

© Swiss Gemmological Institute SSEF

Peridot of 210 ct

from Burma (Myanmar)

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Spinel of 110 ct

from Tajikistan

Kupferhaltige Turmaline aus Mozambique!

Paraiba oder nicht ?, das ist die Frage...

Copper-bearing Tourmaline!


4!

Cause of Colour in Gemstones


1)" Chemical colours, e.g. colouring trace elements (chemical impurity)

2) Structural colours, e.g. defects in the crystal structure

3)" Physical colours, e.g. iridescence, dispersion, diffraction

4) Colours by inclusions

5) Organic colours, e.g. organic pigments

Colour Effects in Gemstones


1) ! Colour zoning

Due to variable uptake of colouring elements during crystal growth.

Colour zoning in Tourmaline

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2) ! Pleochroism

Colour perception varies in different viewing directions

Blue dumortierite from Tanzania


3) ! Colour change effect (also „alexandrite effect)

Different colour perception when viewed under different lighting sources (e.g. greenish in daylight and purple under an incandenscent lamp).

Alexandrite is the most known gemstone showing this effect, but other colour changing stones exist.


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4) ! Colour „change“ when viewed in transmission or reflection

Opal

Due to diffraction of light at submicroscopic dispersed impurities when viewed in front of a light source.

Colour Observation


1)" We can observe colour

2)" We can see what colour is absorbed

3)" We can measure colour

o - rays

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SSEF Photocard


Diamonds

© Photos Christies HK 2010 & Swiss Gemmological Institute SSEF

© G.E. Harlow, 1978!

Even a colourless diamond shows colour, the „fire“ of the diamond is due to dispersion of white light when entering the stone.

Lot 2472: 10.05 ct

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The blue Wittelsbach-Graff Diamond


This historic diamond of 35.56 ct (after recutting 31.06 ct) sold at Christie‘s auction in November 2008 in Geneva for a record breaking 24 mio US$.

Jean-Marc Lunel (Christie‘s Geneva) and Jean-Pierre Chalain (SSEF) with the blue Wittelbach

Yellow Diamonds


colour due to nitrogen impurities

296 ct 325‘296 ct

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Yellow Diamonds


colour due to nitrogen impurities

Less is more for commercial colourless diamonds....

D E F G H I J K L M...

More is better for fancy coloured diamonds....

© V. Pagel-Theisen!

Ruby (variety of Corundum)


Ruby is the red variety of corundum coloured traces of chromium.

Chemically pure corundum (Al2O3) is colourless.

Lot 2382

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The Graff Ruby

© Image courtesy Graff International.

Sold at Christie's in St. Moritz in February 2006 for 3.6 mio US$ ($425,000 per carat).

This record-breaking gemstone was termed the Graff ruby and exhibits the desired "pigeon's blood" red used for highly sought rubies from Burma.

It was also noted that the ruby was certified by the SSEF lab in Switzerland confirming the authenticity of the ruby with “no indications of heating“ and its Burmese origin (Mogok).

New mines


20.46 cts

Excellent ruby of 8 ct from Montepuez in Mozambique.

Exceptional ruby of 20.46 ct from Winza in Tanzania.

Both rubies are unheated and exhibit an impressive purity and a well saturated colour.

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What is ruby ?

-" ruby ?

- pink sapphire ?

- purple sapphire ?


Subtle colour differences for depending on amount of trace elements.

Sapphire of 196 ct from Burma


Blue colour due to traces of iron (Fe2+ and Fe3+)

and titanium.

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Sapphire from Kashmir (approx. 40 ct)

SSEF Appendix letter:


© SSEF Swiss Gemmological Institute

Sapphires from Kashmir contain sub-microscopic fine inclusions which scatter the incomping light. As a result, these stones may show a

highly appreciated velvety blue colour.

© H.A. Hänni, SSEF 2004

The velvety blue of Kashmir sapphires

Lot 2387: 17.45 ct

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Beryl Group


Aquamarine

of approximately 500 ct

Chemically pure beryl is colourless. Various trace elements are responsible for colours such as blue, green, yellow, red, and pink.

Emerald (green variety of beryl)


Very pure emerald from Colombia. No indications of clarity modification.

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Engraved emeralds


118 ct

110 ct

The Cartier Shakti Emeralds: An exceptional pair of emeralds

From the La Pita mine in Colombia, ca. 10 cm in length.

El-Itoco Emerald, 472 ct


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Absorption spectra of ruby (top), alexandrite (middle) and emerald (bottom)

Chromium as colouring element


All three gemstones show similar absorption spectra with characteristic chromium absorption bands.

The density of the crystal structure (ruby = dense; alexandrite = intermediate; emerald = wider) explains why they differ in colour from red to green.

Ruby

Alexandrite

Emerald

Exceptional historical spinel necklace from Tajikistan certified at SSEF


In ancient times, spinel was not known as a mineral and many of the large historic

„Balass rubies“ (e.g. the Black Prince „Ruby“ in the Imperial State Crown of Great Britain) were later identified as spinels.

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Red Spinel from Tajikistan (Pamir)


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

Spinel from Madagascar


Colour changing spinel (84 ct) from Madagascar

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Inclusions in red spinel from Tanzania


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

© H.A. Hänni, SSEF

Luminescence

spectra of spinel


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Jadeite

Chemically pure jadeite is colourless. It belongs to the pyroxene mineral group. !

The most appreciated emerald green jadeite is coloured by chromium and is found in northern Burma.!

© Gemmological Institute SSEF

Fei Cui (Jadeite Jade)

Definition of Fei Cui (Jadeite Jade) "after the Gemmological Association of Hong Kong!

Fei Cui (jadeite jade) is a granular to fibrous polycrystalline aggregate. It is composed of jadeite as major mineral. It may consist of other minor minerals such as opmhacite, kosmochlore, amphibole, and feldspar.!

Kosmochlor (Cr)!

Omphacite (Fe)! Jadeite (Na)!


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Jadeite

Reflections and light diffusion along grain boundaries of polycrystalline jadeite may result in a bright vivid colour.!


SEM BSE picture!

Graphite inclusions may produce greyish and black jadeite-jade!

Natural pearl harvest in Bahrain


Pearl dealer Mr. Abdul Razak Al Mahmood

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Pearl colour


Orient: Iridescence due to light

diffraction and interference

Bodycolour: brown, yellow, crème, orange, pink, grey, black due to organic pigments

Conch pearls (from the Queen Conch)

© Swiss Gemmological Institute SSEF, Verdana, 10pt, normal

Characteristic pink colour and flame-structure

Flame structure, due to densely interlayered aragonite fibres


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© SSEF Swiss Gemmological Institute

Pearls from the Lion paw scallop

Matombo (Uluguru Mountains)"

Morogoro Province!


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Low temperature heating (<1000 °C), e.g. Mong Hsu rubies and pink sapphires effect: colour shift, blue colour component is reduced

Ruby treatment Verdana 24pt, bold


Gemstone market in Ratnapura

Thank you for your attention

Swiss Gemmological Institute SSEF

website: www.ssef.ch email: [email protected]

Gemstones The Fascination of Colour€¦ · 1! Presentation by Dr. Michael S. Krzemnicki Swiss...

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Transcript of Gemstones The Fascination of Colour€¦ · 1! Presentation by Dr. Michael S. Krzemnicki Swiss...