Gem-Session 3 “Inclusions”

January 29, 2012 | No Comments

Gemstones are beloved and costly, not only because of their colour and play of light within, but also because of their durability and rarity. Inclusions in impressive gemstones are often  seen as ‘pollution’ or as a ‘blot’, but to gemmologists inclusions may tell so much about a gemstone and even in unique situations they are the true added value to a stone.
Inclusions are not ‘just trash’ in a stone besides differentiating the value between flawless diamonds and diamonds with small inclusions, they confirm the authenticity of a stone and can sometimes make a stone even unique. Think about the exceptional appearance of an emerald, wherein the specific area with the typical inclusions is expressed by ‘le jardin’ ( ‘the garden’).
To give you an impression of information that inclusions can give about the authenticity and origin of the stone I will give you the example of diamond, text & photos by Hanco Zwaan .
Inclusions and the origin of diamond
Diamond is a crystallised form of carbon that in principle can only be formed under high pressure (and heat) and therefore can only originate within the earth, at depths of 150 kilometres or more in the upper part of the earth mantle (with typical ideal pressure and temperature: 60 kilo bar and 1000 degrees Celsius). At lower pressure, carbon will crystallise as the mineral graphite.
Diamonds can be formed in various types of mantle rocks, of which ‘harzburgite’ and ‘eclogite’ are the two most important ones. The crystals in photo 1A and B are both olivine inclusions, which are often seen in diamonds. These inclusions show that the specific diamond is formed in harzburgite, which largely consists of olivine. Not only do these inclusions tell that the diamond is formed at great depth in the earth’s mantle but they also confirm that the diamond is very old. Dating on inclusions on diamonds is formed in harzburgite revealed ages mostly around 3 billion years old. Diamonds formed in eclogite are usually much younger.
Diamonds are not stuck in the deep mantle. They are driven upwards together with magma (molten rocks) and therefore they are now found close to the earth’s crust in unique/rare volcanic rocks, like kimberlite or lamproïte. These rocks are far younger than the diamonds, but are situated in very old continental deposits. For example, like in South-Africa and Australia, where amongst them exists mantle rocks of more over than 2,5 billion years old.
For the diamond to traverse its way to the earth’s crust, a volcanic eruption must have occurred wherein the magma with high speed is pushed upwards through cracks of the earth’s crust. In case of kimberlite the distance is more than 150 kilometres, by other magma’s less than 60 kilometres. Herewith the ‘surroundings pressure’ around the traversed diamond decreased. During decreasing pressure olivine will extend faster than diamonds. This causes a lot of stress within the diamond that leads to many stress releasing cracks in the surrounding of the olivine inclusions. These cracks could never healed perfectly, because under less pressure it’s impossible to form diamond, but only graphite. That’s why the cracks are coloured black. This process is called internal graphitisation in a diamond.

Summing up the inclusions of photo 1a and 1b give a unique view of time and place wherein this diamond is formed and show the speed in which the diamond was transported from great depth into the direction of the earth’s surface.

For more beautiful immages of inclusions in diamonds, please take a look at ‘All About Gemstones’.Another totally different great example  is Amber
It is important to first note that gems can be divided into organic (like coral, jet, amber, pearls, ivory etc.)  and inorganic (crystalline gemstones) materials. Amber, a fossilised resin, and the younger variety Copal are sometimes filled with interesting insects.

Lately scientists discovered a piece of amber originating from Canada more over than 70 million years old and probably capturing dinosaur and bird feathers! The scientists described the collection in ‘Science Magazine’, where you can buy the download of the article. For a simple  overview of different stages of trapped dinosaur feathers in amber I would advice you to visit the photo stream of  Discover Magazine . 

If you would like to learn more about gemstones visit the site of Gem-A, they frequently organise events, workshops & seminars. 

Museum recommendation:
The mineral and gemstone collection at the Natural History Museum in London.
Serpentina ‘Die Schlange im Schmuck der Welt’ (Snake Jewellery from around the World), Schmuck museum Pforzheim. Until 26th February 2012.

Book recommendation:‘Gemstones, Understanding, Identifying, Buying’, by Keith Wallis. Hardback & 1000 colour illustrations. ISBN: 9781851496303, $20.

Acknowledgement:
I would like to thank Dr. Hanco Zwaan of The Netherlands Gemmological Laboratory for his valuable input and suggestions, and for the use of the microphotosgraphs of the diamonds.

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About

Margriet Sopers

Margriet Sopers, FGA

Having a diploma in jewellery, FGA and a propaedeutic of the Academy of Art followed-up by interesting years as a jewellery expert at Sotheby's & Gemeentelijke Krediet Bank, as well as being a member for years of the Society of Jewellery Historians, I am glad to share with you today's world of jewellery.

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