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Organic Gems Part IV: Ammolite

Hundreds of millions of years ago, the Earth was very different. Little did the creatures of our planet know, they would not only be a stepping-stone in the evolution of life, but also provide the humans of the future with fabulous jewellery gems in the form of ammolite.

The process begins with ammonites, marine invertebrate animals that are now extinct. They thrived in tropical seas during the Devonian geological period, beginning about 400 million years ago, to the close of the Cretaceous period 66 million years ago.

Ammolite forms when the fossilised shell of the ammonite is preserved and the cavities that originally held its soft body are filled with aragonite – the same mineral that is responsible for the nacre of pearl oyster shells.

The resulting organic gem – which resembles an opal in some ways – is beautifully coloured, with a wonderful iridescent sheen.

The ammonites that form ammolite specifically inhabited a prehistoric inland subtropical sea that bordered the Rocky Mountains of northwest America.

As the sea receded, layers of sediment preserved the shells.

Ammonite shells comprise a number of minerals including small aragonite platelets, and depending on the conditions of deposition, these are often replaced by pyrite or calcite.

"The pattern, intensity and range of colour all contribute to the overall value of an ammolite gem. Green and red are the most common colours, with blue and violet being rarer, and therefore more valuable"

The result is a pseudomorph of the original shell shape, which may contain beautiful cavities of crystalline material throughout the shell‘s structure. Thus ammonites themselves can make beautiful and interesting pieces of jewellery.

But it is the ammolite that is the most desired and collectable form of these ancient creatures.

The vivid iridescent sheen of ammolite is caused by an interference effect, when white light is refracted and reflected back from the layered aragonite platelets within the gem’s structure. The thicker these layers, the more red and green hues are seen; when layers are thinner, violet and blue hues dominate.

The pattern, intensity and range of colour all contribute to the overall value of an ammolite gem. Green and red are the most common colours, with blue and violet being rarer, and therefore more valuable.

Ammolite may be described as either fractured or sheet. Sheet ammolite is unbroken, with a continuous movement of colour across its surface. Fractured ammolite may have various different patterns and some have been described with terms such as ‘dragon skin’, ‘cobblestone’, ‘moonglow’ and ‘paint brush’.

Because the ammolite layer of the shell is usually mere fractions of a millimetre in thickness, most ammolite gems are in fact composite stones, generally in the form of a doublet or triplet. The ammolite is adhered to a dark backing material, usually its matrix or mother rock.

As the ammolite is thin and fragile, a second layer, generally a polymer, is added to protect and stabilise the stone. To enhance the optical display, a piece of synthetic spinel, quartz or glass-type material may be placed on top.

Ammolite is a magnificent gemstone with an incredible history of creation over millions of years. It is one of the most rare and beautiful organic minerals for use in jewellery.











ABOUT THE AUTHOR
Stacey Lim

Contributor • Registered GAA Gemmologist & Valuer


Stacey Lim FGAA BA Design, is a qualified gemmologist and gemmology teacher/assistant. She is a jewellery designer, marketing manager and passionate communicator on gemmology. For information on gemstones, visit: gem.org.au

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Monday, 26 August, 2019 03:28pm
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