Nuummite – the Origin of Colour

This article by Guy Lalous ACAM EG summarises a technical article from The Journal of Gemmology discussing the origin of colour and conditions of formation in violet-to-blue Nuummite from Simiuttat, Greenland. 

Nuummite is an iridescent orthoamphbiole rock found in the Nuuk Distruct in south-west Greenland. It has only been known in the trade since 1983. However, the rock is believed to have been formed in the Late Archean period more than three billion years ago. 

A study published in The Journal investigates the mechanism for the violet-to-blue diffraction colouration of the Simiuttat Nuummite, providing a petrological description of the rock and highlighting the metamorphic pressure-temperature (PT) conditions of its formation. The study concludes that the colour of Nuummite is not due to grain chemical variation, but to the spacing of exsolution lamellae in orthoamphibole.

Some gemstones are formed by microscopic layers of material that are stacked upon one another. When light interacts with the different stacks within the gem material it causes flashes of colour. This is called iridescence, which refers to the scattering and diffraction of light, the iridescent effect is seen in a few gemstones including labradorite, moonstone and Nuummite.  

Checkerboard-cut Nuummite gemstone, 57.73 ct showing long, thin, violet-to-blue prisms. Image courtesy of Tay Thye Sun.
Checkerboard-cut Nuummite gemstone, 57.73 ct, showing long, thin, violet-to-blue prisms. Image courtesy of Tay Thye Sun.

What is the difference between iridescence and diffraction?

Iridescence happens when light passes through a thin, transparent film with a different refractive index from the surrounding material. Thin-film interference of visible light occurs when light interacts with a lamellar material consisting of fine layers that have different refractive indices (RIs). In diffraction, light waves pass through a grating. Diffraction is caused by the bending, spreading and subsequent overlapping of a wave-front when passing through a tiny opening or openings in an otherwise opaque obstacle. Iridescence is frequently used to describe any diffraction and/or thin-film interference-related colour phenomena, as interference and diffraction in minerals are closely related and often occur together.  

Read more: Reconstructed Amber Broken Down

The RIs measured on a polished slice of Nuummite were 1.650–1.660, with a second shadow edge at approximately 1.54. Hydrostatic specific gravity (SG) measurements were 3.09 ± 0.01. The presence of amphibole-, biotite- and cordierite-rich areas explains the different refractometer readings, with values of about 1.66 being typical for orthoamphibole while the additional shadow edge of 1.54 originated from cordierite.  

Observation of the polished slice with a digital microscope showed laths of orthoamphiboles criss-crossing one another. The crystals displayed blue-to-violet flashes, which were only visible on {210} cleavages. The colours vanished when the crystal was tilted to an angle of 5–15° depending on the tilting axis, indicating a strong geometric control on the colouring phenomenon. The surface topography of the {210} cleavages might act as a diffraction grating that could contribute to the iridescence.  

Keyence digital microscope photos of a polished slice of Nuummite show criss-crossing orthoamphibole prisms in a dark matrix of cordierite and biotite. Image courtesy of L. Franz.
Keyence digital microscope image: polished Nuummite showing criss-crossing orthoamphibole prisms in a dark matrix of cordierite and biotite. Image courtesy of L. Franz.

Polarised microscopy of the thin section revealed a close inter-growth of prismatic orthoamphibole, granular cordierite (with weak pinitisation) and brown, strongly pleochroic biotite flakes. All minerals displayed a random orientation, the correct petrographic name of the investigated Nuummite is biotite-cordierite-anthophyllite granofels.   

What is a granofels?

A granofels is a metamorphic rock with prevailing granular-textured minerals lacking any alignment. The anhydrous mineral assemblages found in granofels are produced at the base of the crust, where the conditions lead to their formation. 

Read more: Gem Central Exploring Ruby Treatments with Julia Griffith FGA DGA EG

Phase diagram calculations show the stability field for the mineral assemblage at temperatures of 505-660°C and pressures below 6.4 kbar. The electron microprobe analysis (EMPA) data of the orthoamphibole crystals from the Nuummite samples plot at 590–600°C (±25°C) in the T–AlIV solvus diagram of Spear, constraining the minimum metamorphic temperature experienced by the rock.   

What is electron microprobe analysis (EMPA)? 

Electron microprobe analysis is an analytical technique that is used to establish the composition of small areas of a specimen. The method is non-destructive and utilises X-rays excited by an electron beam, incident on a flat surface of the sample. 

What do we know about the solvus diagram of spear?

Solvus thermometry involves phases that form a solid solution at high-T but that 'unmix' into separate phases during cooling. The composition of coexisting minerals with a solvus relationship is an indicator of temperature. Data on mineral assemblages can be obtained through EMPA. The existence of a solvus in the orthoamphiboles between low-Al anthophyllite and high-Al gedrite has been noted by several scientists decades ago. Frank S. Spear (1980) conducted research on orthoamphiboles. His conclusion was that careful declination of the solvus T-X space enables to estimate temperatures in samples where two orthoamphiboles coexist.    

Read more: Zircon from Vietnam: Properties and Heat Treatments

EMPA and Raman spectroscopy classify the orthoamphibole as an Al-rich anthophyllite. Raman spectroscopy of the orthoamphibole showed great similarities to gedrite in the RRUFF database.  

TEM image. Image courtesy of R. Wirth.
L: TEM image of violet-to-blue orthoamphibole shows alternating lamellae of anthophyllite and gedrite
R: In the electron diffraction pattern from the orthoamphibole shown (L), each spot along the crystallographic b-direction is split (e.g. see arrows).  Images courtesy of R. Wirth.

Transmission electron microscopy (TEM) images showed a continuous succession of alternating exsolution lamellae of anthophyllite (wide, dark grey bands) and gedrite (narrow, light grey lamellae), which also were revealed by analytical electron microscopy (AEM) chemical analyses. The exsolution lamellae may have formed during cooling after peak metamorphism, during reheating processes in the course of the intrusion of a granite complex or during later metamorphic overprint.  

What is transmission electron microscopy (TEM)?

TEM operates on the same basic principles as a conventional microscope but uses electrons instead of light. What can be seen with a conventional microscope is limited by the wavelengths of light. The much lower wavelengths of the electrons allow a resolution that is thousand times better than with a light microscope.

What is analytical electron microscopy (AEM)?

AEM refers to the collection of spectroscopic data in TEM based on various signals generated following the inelastic interaction of the incident electron beam with the sample. These signals can be used to identify and quantify the concentration of the elements present in the analysed area, map their distribution in the sample with high spatial resolution (down to 1 nm or better), and even determine their chemical state.

Mineral composition thin section Nuummite. Image courtesy L. Franz
Mineral composition thin section Nuummite with L: parallel polarisers R: crossed polarisers. Revealing randomly orientated orthoamphibole (oam), cordierite (crd), biotite (bt) and accessory ilmenite (ilm). Image courtesy L. Franz.

Conclusion

The iridescence of Nuummite is due to the interference of light reflected from sub-microscopic, alternating gedrite and anthophyllite exsolution lamellae. Average spacing of 124-133 nm between the lamellae will generate violet-to-blue diffraction colouration, a previous study indicated an average spacing of 180 nm gives rise to yellow iridescence. 

Acknowledgments

We sincerely thank Rex Guo for donating material for this research. We owe great thanks to Dr Nynke Keulen, Karsten Secher and Peter Appel of the Geological Survey of Denmark and Greenland (Copenhagen, Denmark) for providing information on the occurrence of Nuummite in Greenland. Thanks also to Willi Tschudin (Universität Basel) for preparing the thin section and polishing the slice of Nuummite. Finally, thanks to Anja Schreiber (GFZ Potsdam) for the careful preparation of the TEM slices. The manuscript was considerably improved by the suggestions of three anonymous reviewers. ■ 

This is a summary of an article that originally appeared in The Journal of Gemmology titled ‘Violet-to-Blue ‘Nuummite’ from Simiuttat, Greenland: Origin of Colour Appearance and Conditions of Formation’ by Leander Franz, Tay Thye Sun, Richard Wirth, Christian de Capitani and Loke Hui Ying  2016/Volume 35/ No. 4 pp. 330-339 

Interested in finding out more about gemmology? Sign-up to one of Gem-A's courses or workshops.

If you would like to subscribe to Gems&Jewellery and The Journal of Gemmology please visit Membership.

Cover image Keyence digital microscope image of polished Nummite to show an orthoamphibole prism displaying violet-to-blue colouration. Image courtesy of L. Franz. 


The Fascinating History of Antique Turquoise Jewellery

The Fascinating History of Antique Turquoise Jewellery

In his third Gemstone Conversations column for Gems&Jewellery, Jewellery Historian and Valuer John Benjamin FGA DGA FIRV explores the fascinating history of turquoise and its use in jewellery design from the Shahs of Persia to the Art Deco design movement.

Read more


Birthstone Guide: Garnet For Those Born In January

Birthstone Guide: Garnet For Those Born In January

If you're lucky enough to be born in January, vibrant garnet is your birthstone. A rainbow jewel of the gem world, garnet displays the greatest variety of colour of any mineral and is very often untreated, making it a rarity in the gem world. 

Read more


Getting Started with Quartz Inclusions

Getting Started with Quartz Inclusions

Do you know your calcite inclusions from your dumortierite, epidote, fluorite and rutile? Here, Charles Bexfield FGA DGA EG explores some incredible quartz inclusions and explains what to look for when shopping for quartz specimens.

Read more


Understanding Iridescence: Opals, Pearls, Moonstones and Fractured Stones

Understanding Iridescence: Opals, Pearls, Moonstones and Fractured Stones

Iridescence has to be one of the most mesmerising and magical optical effects seen in gemstones. But have you ever wondered how it occurs? Gem-A's Collection Curator Barbara Kolator FGA DGA shines a light on this fascinating optical effect and tells us about the gems that are most likely to display it.

Read more


Hidden Treasures: Highlights of Gem-A's Gemstones and Minerals Collection

Hidden Treasures: Highlights of Gem-A's Gemstones and Minerals Collection

Gem-A Gemmology Tutor Pat Daly FGA DGA offers us a glimpse at some of the more unusual items in Gem-A's Gemstones and Minerals Collection.

Read more


Tanzanite: The Contemporary December Birthstone

Tanzanite: The Contemporary December Birthstone

Are you looking for the perfect festive gift for a December baby? Gem-A tutor Lily Faber FGA DGA EG considers tanzanite – one of three birthstones for December – and shares how this relatively new gemstone compares to its purple and blue-hued rivals.

Read more


Birthstone Guide: Turquoise For Those Born In December

Birthstone Guide: Turquoise For Those Born In December

Beautiful blue turquoise is one of three birthstones for the month of December (in addition to zircon and tanzanite). It is enriched with real cultural significance that can be traced back thousands of years. Here, we explore the blue shades of turquoise and explain what makes this gemstone so special...

Read more


Understanding the Cat's Eye Effect in Gemstones

Understanding the Cat's Eye Effect in Gemstones

Chatoyancy is the gemmological name given to the curious optical effect in which a band of light is reflected in cabochon-cut gemstones, creating an appearance similar to light bouncing off a cat's eye. Gem-A's Collection Curator, Barbara Kolator FGA DGA explains chatoyancy and highlights some of the many gems in which it can occur.

Read more


Jade and its Importance in China

Jade and its Importance in China

Jade has long been revered by gem lovers internationally, but nowhere more so than in China. But what is it that makes this gemstone so special? Gem-A's Assistant Gemmology Tutor Dr Juliette Hibou FGA gives us an overview of jade, how to identify it and its significance in Chinese culture.

Read more


Highlights of Gem-A Conference 2019

Highlights of Gem-A Conference 2019

The Gem-A Conference is always the highlight of our gemmological calendar! If you didn’t manage to make it, we’ve put together a few of the highlights from this year’s event to fill you in on what you missed, and whet your appetite for Gem-A Conference 2020!

Read more