October 26, 2020

Peter Rosewarne

Polished slabs and spheres don’t usually figure highly in my wish-list of mineral specimens but, over the years, some colourful and interesting ones have caught my eye and have been added to the Rosey Collection. This short article highlights some of what I hope you will agree are both unusual, interesting and colourful polished slabs from various localities around the world. The slabs very briefly described and illustrated herein are Sonora Sunrise and Laguna Agate from Mexico, Schalenblende from Poland, variscite from the USA, eclogite from South Africa, Condor Agate from Argentina, and prehnite and Sunset Jasper from Australia. Two more common minerals are included at the end for interest.

This article concentrates on the aesthetics of each slab, illustrated by photographs, with details of locality but only minimal information is given on mineralogy and no dimensions. Snippets of information on source areas and minerals have been gleaned from the Internet and books but not in sufficient detail to warrant a list of references.

Sonora Sunrise

This rock is also known as Sonora Sunset but for reasons that will become apparent when you read about the last ‘unusual’ slab listed in this article, I’ve used Sunrise here. It also fits with this being the opening description in this article. This is a very colourful slab of green chrysocolla separated from a central area of orange-red cuprite by a zone of black manganese oxide. It comes from the famous Milpillas Mine (famous for inter alia world-class specimens of azurite, malachite, brochantite and cuprite) in Sonora, Mexico. I acquired this specimen from Douglass Minerals, attracted by its unique colour combination.

Laguna Agate

Laguna Agate is apparently one of the most prized agates in the world owing to its tight banding and bright colours. It is found almost due south of El Paso in the state of Chihuahua, Mexico and is mined in a mountainous area associated with andesitic lavas. This piece was acquired from Hummingbird Minerals and lives up to its reputation, I think?


This is probably one of the strangest looking examples of sphalerite that you’ll ever see. It’s mainly a botryoidal (colloform, reniform – take your pick) combination of sphalerite (brown and yellow – the latter possibly wurtzite), marcasite and galena (metallic grey). It’s from the Olkusz Mine east of Katowice in Poland. Schalenblende is thought to have originated from rapid crystallisation of a sulfide gel and the name is derived from the German for shale ore.


Variscite is a light green/turquoise-coloured hydrated aluminium phosphate mineral formed by deposition from phosphate‑bearing water reacting with aluminium-rich rocks. It normally forms as nodules and a notable occurrence is Fairfield in Utah, USA. This specimen comes from the Little Green Monster Variscite Mine in that area and was acquired from Dakota Matrix Minerals. The light-coloured mineral is a calcium aluminium phosphate called crandallite and this may be the first and last time it ever gets a mention in MinChat!

Eclogite Figure 5 
The slab in Figure 5 was acquired from Richard Harrison as part of a trade for a very nice N’Chwaning rhodochrosite. It’s from a nodule in kimberlite at the Rovic Diamond Mine, now closed, near Boshof in the Free State. I’ve also got a rough piece and a polished sphere of this material but what sets this slab apart from your ‘normal’ eclogite is the presence of accessory kyanite within the primary omphacite pyroxene and pyrope garnet.

Figure 6
This can be seen as small blue patches in Figure 6. This eclogite is rather dull in comparison with some examples with bright green omphacite and red garnets, as shown in Figure 7.

Figure 7
This specimen was acquired from Jo Wicht, in a trade for a nice yellow Moroccan fluorite with red quartz, but its provenance is unknown and it may even be leaning more towards a garnet peridotite as there appear to be two green minerals present, one showing dark fracture or cleavage lines more typical of olivine. But who cares; it’s beautiful! Eclogites deserve a whole article to themselves, but not here.

Condor Agate
This is probably the least aesthetic of the pieces described herein but the name has a ‘ring’ to it. This apparently popular agate was discovered in 1993 at San Rafael, Mendoza Province in the Patagonia region of Argentina. It is mined from shallow surface excavations. This one has a quartz centre. Acquired from the Cal Neva Mineral Company.


I bought this piece in Perth, Australia in 2015 attracted both by the colour and the difference in habit compared to the more familiar material from Namibia. It originates from Wave Hill in the Northern Territories, the source of the finest gem-grade prehnite in the world. It occurs in basalt and varies from green to yellow and goes under the trade name of SunJade®.

Sunset Jasper

A fitting name to end this part of the article. This rock was also bought in Perth at the same time as the prehnite and I’m still kicking myself for not rather or additionally buying a slab of Pilbara tiger’s eye which has a lovely reddish hue that doesn’t seem to be present in the South African tiger’s eye. Anyhow, not much to say about this one apart from that it originates from the Hamersley iron ore mining area in the Pilbara Region of Western Australia and brings down the curtain on the first part of this brief article on unusual polished slabs.

However, two additional small polished slabs of rhodochrosite and malachite are included below for their aesthetics to round-off this article, as they are not unusual minerals. The only unusual aspect is that, in my experience, most rhodochrosite and malachite specimens are either raw mineral specimens or polished carvings/ornaments, eggs or spheres, so perhaps they qualify as unusual polished slabs?

Rhodochrosite (Figures 11 and 12)

Figure 11 is a section through a stalactite/stalagmite (my ‘A’ level geology teacher taught us to remember that tites come down and mites go up) from the famous Capillitas Mine in Argentina. It was acquired from John Betts Fine Minerals. This mine is located at an altitude of about 3 000 m and has been worked since Inca times and hence the earlier name given to the mineral, Inca Rose. Unlike the crystalline occurrences at e.g. N’Chwaning (scalenohedrons) in South Africa and the Sweethome Mine (rhombohedrons) in the USA, Capillitas rhodochrosite is almost entirely formed as stalactites and stalagmites and massive deposits (e.g. Figure 12, from UC Minerals). The site was originally mined for sulfides but these have been worked-out. 

This one isn’t labelled but I recall winning it in an online auction and that it is from the DRC and is also from a stalactite/stalagmite but that’s about all I can say about it. And that really does bring down the final curtain on this article. PR


October 26, 2020

Lesley Andrews

I am the proud owner of two stone plants which I keep on the stoep table - these are decorated by a surface layer of small tumbled semi-precious stones. Recently I was astonished to see a threesome of Cape Turtle Doves on the table carefully picking out some of the stones, passing them to each other, rolling them around in their beaks and putting them down carefully all over the table. I knew that many birds eat grit, but why this preference for my ornamentals?

The Cape Turtle Do...

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September 26, 2020

I do give myself complications.

Based on the success of the “September Spheres”, we invited photos of “royal blue” minerals for our October newsletter. This is because we are featuring Peter’s detailed article on lapis lazuli. Ultramarine is such a rich colour and there are not that many minerals of such a classic blue.

For my contribution, I photographed the few possibilities I had in my mineral cabinet, but thought a bit more. There was an odd offcut of stone in the outside cupb...

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September 26, 2020

I first visited the Tucson shows in 1992. My intention was to buy faceting rough, but there was almost none I could afford, although the rand/dollar exchange rate was 6:1. One day, walking around with my friend and research colleague David Killick from the University of Arizona we wandered around, dazed and bewildered by the spectacular minerals from Afghanistan on display in one of the numerous tented venues. On one table, crowded with Afghan ethnic jewellery, possibly all modern, there sat ...

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September 25, 2020

Peter Rosewarne

And now for something completely different, from me at least. I don’t normally write about semi-precious ornamental stones/rocks but felt there was a story in this one based on a long-ago overseas trip, a more recent article in the Mineralogical Record, some carvings I have from the former and some mineral specimens related to the latter.

Firstly, some technical clarity about lapis lazuli, or ‘lapis’, which many of you probably don’t need. I had always thought that ...

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August 24, 2020
Duncan Miller

It’s play time!  For this exercise you need a flat computer screen with an open blank Word page, a smallish clear quartz crystal, a quartz sphere if you have one, or if not, some clear quartz beads (glass beads won't work), plus a pair of cinema 3D glasses or Polaroid sunglasses. 

Quartz crystals are anisotropic. This means that a ray of light travelling through the crystal is split into two polarised rays, vibrating at right angles to each other. There is only one directio...

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Stories Behind Some Recently Acquired ‘Exotic’ Rocks and Polished Spheres

August 23, 2020

by Peter Rosewarne

With the lockdown in force it’s given me some time to revisit my passion for igneous rocks and their minerals, being what we used to call a ‘min and pet’ man whilst studying geology at Kingston University back in the early 70s. Of particular interest to me on the local scene are the Bushveld Igneous Complex, the Pilanesburg Alkaline Complex, kimberlites, ultramafics and the Vredefort Dome.  In my quest to find specimens of the ‘type’ rocks from these sites, in a...

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July 27, 2020

Duncan Miller

This month we are going to do all three, recycle an old publication, reuse it with additional photographs, and repurpose it as an article on the club’s website. The article describes Cape Town’s former tin mines, and the website article has a virtual tour of the Vredehoek tin mine on Devil’s Peak, courtesy of Dr Gregor Borg of Halle University in Germany. All of this is available for download from

Few people know that ...

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July 27, 2020
Duncan Miller

Olivine is the name given to both a mineral and a mineral group. The mineral olivine is an orthorhombic silicate with the relatively simple chemical formula of (Mg,Fe)2[SiO4]. It has a continuous range of composition between two end members, one magnesium-rich and the other iron-rich. The magnesium end member of the range is called forsterite (Mg2SiO4) and the iron end member is fayalite (Fe2SiO4). These distinct minerals form part of the olivine group, which also includes a mang...

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July 27, 2020

Lesley Andrews

Peter Carl Fabergé was born in St Petersburg in 1846, and eventually became Goldsmith to the Imperial Court of Russia, and a supplier of wonderful artworks worldwide. He assumed charge of the workshop his father had established in St Petersburg when he was only twenty-four years old, and presented the first Imperial Easter Egg to the Russian royal family in 1884. The workshop survived until 1918, by which time Russia had succumbed to revolution and the royal family was no mor...

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