Gemstones and the fascination of colour (2010)

Christie’s Spring Auctions 2010 Lecture Sunday 30th May 2010

Gemstones & The Fascination of Colour Presentation by Dr. Michael S. Krzemnicki Swiss Gemmological Institute SSEF Photos © M.S. Krzemnicki, SSEF, except where indicated otherwise

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Colour... © Swiss Gemmological Institute SSEF!

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...is emotion © Swiss Gemmological Institute SSEF!

Peridot of 210 ct from Burma (Myanmar)

© Swiss Gemmological Institute SSEF

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Spinel of 110 ct from Tajikistan

© Swiss Gemmological Institute SSEF

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Kupferhaltige Turmaline Copper-bearing Tourmaline aus Mozambique

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

© Swiss Gemmological Institute SSEF

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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 © Swiss Gemmological Institute SSEF

Colour Effects in Gemstones 1)! Colour zoning Due to variable uptake of colouring elements during crystal growth.

Colour zoning in Tourmaline © Swiss Gemmological Institute SSEF

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Colour Effects in Gemstones 2)! Pleochroism Colour perception varies in different viewing directions

Blue dumortierite from Tanzania © Swiss Gemmological Institute SSEF

Colour Effects in Gemstones 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.

© Swiss Gemmological Institute SSEF

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Colour Effects in Gemstones 4)! Colour „change“ when viewed in transmission or reflection Due to diffraction of light at submicroscopic dispersed impurities when viewed in front of a light source.

Opal

© Swiss Gemmological Institute SSEF

Colour Observation 1)" We can observe colour 2)" We can see what colour is absorbed

3)" We can measure colour o - rays

© Swiss Gemmological Institute SSEF

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SSEF Photocard

© Swiss Gemmological Institute SSEF

Diamonds

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

© G.E. Harlow, 1978!

© Photos Christies HK 2010 & Swiss Gemmological Institute SSEF

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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 © Swiss Gemmological Institute SSEF

Yellow Diamonds colour due to nitrogen impurities

296 ct

325‘296 ct

© Swiss Gemmological Institute SSEF

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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...

© V. Pagel-Theisen!

More is better for fancy coloured diamonds....

© Swiss Gemmological Institute SSEF

Ruby (variety of Corundum) Chemically pure corundum (Al2O3) is colourless.

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

Lot 2382

© Swiss Gemmological Institute SSEF

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The Graff Ruby 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). © Image courtesy Graff International.

New mines

Excellent ruby of 8 ct from Montepuez in Mozambique.

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

20.46 cts

Exceptional ruby of 20.46 ct from Winza in Tanzania.

© Swiss Gemmological Institute SSEF

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What is ruby ?

Subtle colour differences for depending on amount of trace elements.

-" ruby ?

- pink sapphire ?

- purple sapphire ?

© Swiss Gemmological Institute SSEF

Sapphire of 196 ct from Burma

Blue colour due to traces of iron (Fe2+ and Fe3+) and titanium. © Swiss Gemmological Institute SSEF

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Sapphire from Kashmir (approx. 40 ct)

SSEF Appendix letter:

© Swiss Gemmological Institute SSEF

The velvety blue of Kashmir sapphires 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.

Lot 2387: 17.45 ct © SSEF Swiss Gemmological Institute

© H.A. Hänni, SSEF 2004

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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.

© Swiss Gemmological Institute SSEF

Emerald (green variety of beryl)

Very pure emerald from Colombia.

No indications of clarity modification. © Swiss Gemmological Institute SSEF

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Engraved emeralds The Cartier Shakti Emeralds: An exceptional pair of emeralds

118 ct

110 ct © Swiss Gemmological Institute SSEF

El-Itoco Emerald, 472 ct From the La Pita mine in Colombia, ca. 10 cm in length.

© Swiss Gemmological Institute SSEF

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Chromium as colouring element Absorption spectra of ruby (top), alexandrite (middle) and emerald (bottom) All three gemstones show similar absorption spectra with characteristic chromium absorption bands.

Ruby

Alexandrite

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

Emerald

© Swiss Gemmological Institute SSEF

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.

© Swiss Gemmological Institute SSEF

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Red Spinel from Tajikistan (Pamir)

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

Spinel from Madagascar

Colour changing spinel (84 ct) from Madagascar © Swiss Gemmological Institute SSEF

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Inclusions in red spinel from Tanzania

© H.A. Hänni, SSEF

Oriented planes of fine „dust“ precipitates in spinel from Tanzania © Swiss Gemmological Institute SSEF

Luminescence spectra of spinel

© Swiss Gemmological Institute SSEF

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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)!

© Swiss Gemmological Institute SSEF

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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! © Swiss Gemmological Institute SSEF

Natural pearl harvest in Bahrain

Pearl dealer Mr. Abdul Razak Al Mahmood

© Swiss Gemmological Institute SSEF

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

Orient: Iridescence due to light diffraction and interference © Swiss Gemmological Institute SSEF

Conch pearls (from the Queen Conch)

Flame structure, due to densely interlayered aragonite fibres

Characteristic pink colour and flame-structure © Swiss © Swiss Gemmological Gemmological Institute Institute SSEF, SSEF Verdana, 10pt, normal

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Pearls from the Lion paw scallop

© SSEF Swiss Gemmological Institute

Matombo (Uluguru Mountains)" Morogoro Province

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© Swiss Gemmological Institute SSEF

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Gemstone market in Ratnapura Ruby treatment Verdana 24pt, bold Low temperature heating (<1000 °C), e.g. Mong Hsu rubies and pink sapphires effect: colour shift, blue colour component is reduced

© Swiss Gemmological Institute SSEF

Thank you for your attention

Swiss Gemmological Institute SSEF website: www.ssef.ch email: gemlab@ssef.ch

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