TRIALS AND TRIBULATIONS WITH CUT GEMSTONE HEATING

March 25, 2021
Duncan Miller

Inspired by the dramatic change in colour of the large tourmaline illustrated in last month’s Mineralogical Chatter, that went from autumn brown to a purplish-pink on heating by the client for whom I had cut it, I decided to experiment myself. A friend lent me a small ‘enamelling’ kiln; I bought a suitable crucible from jewellers’ supplier Lipman & Son in Cape Town (https://lipmanson.co.za/); and Ian Lipman generously gave me jewellery casting investment powder to protect the stones. My first experiment was to find out how the kiln’s temperature controller worked and how long it took to reach the required temperatures. I also had to research what those temperatures were for different gemstones. An enquiry to GemologyOnline led to a longish online conversation, with detailed advice from some very experienced lapidaries and gemstone dealers (https://www.gemologyonline.com/Forum/phpBB2/viewtopic.php?f=8&t=26065).

My first experiment was on a small, dark red tourmaline that looked like a garnet. It was buried in two tablespoons of investment powder in the crucible and heated in one step to 500 °C. It took only 15 minutes to reach this temperature, overshot to 535 °C, and then settled down for a one-hour soak at 500 °C. Obviously the kiln control would have to be nursed when approaching a set temperature. The kiln was left to cool overnight before removing the stone. The tourmaline had changed colour to a bright pink, but the internal flaws became more visible and the facets all developed scattered pitted areas. I thought that may be due to some reaction with the investment powder.


Then I tried more controlled heating of three khaki-brown tourmalines, taking the kiln up in steps of 200 °C per hour, with a one-hour soak at 500 °C. Two of the stones were wrapped in aluminium foil to isolate them from the investment powder. The third was ‘naked’ to see if it developed surface damage. It didn’t, and all three stones changed to a not very appealing yellowish-green with serious internal crazing. It was clear from these very limited tests with tourmaline that heating has a low potential of success unless the stones are free from inclusions and flaws to start with.

The owner of the kiln had given me two pairs of light greenish-blue aquamarines to heat to try to remove the green cast. I only heated one pair, to keep the other for comparison. The pair of stones when heated slowly to 450 °C and cooled overnight lost the green and became a more pure blue, but they were pale to start with so the change was not dramatic. They didn’t suffer any damage, which was a pleasant relief after the lack of success with the tourmalines.

My experiment with heat-treating dark amethyst was a bit more successful. Many years ago I had lightened dark Namaqualand amethyst in a kiln at UCT, but had forgotten the temperature used. A quick search on the internet produced a link to a recent, detailed, open-source article on the effects of heating amethyst (https://www.nature.com/articles/s41598-020-71786-1). So I buried four stones in the dry investment powder and set the kiln controller to 300 °C, knowing that it would overshoot that mark. It reached 360 °C, which was the actual temperature I was aiming for, and then I switched it off to cool overnight. The result was that all four stones lightened slightly. Three stones suffered no damage, while the larger oval developed visible internal cracks that were not present originally. But they were still rather dark, so they were heated again to 385 °C. The additional 25 °C made a bigger difference. Interestingly, all the stones did not lighten to the same degree. Even different portions within some individual stones responded differently.

My conclusion from reading about heat treatment of gemstones online and this brief experiment is that it is a somewhat hit-and-miss business with a substantial failure rate. The advice of experienced heat treaters is that even gem stones from the same source may respond differently to heating, with rather unpredictable outcomes. I have satisfied my curiosity, and don’t plan on doing any more gemstone heat treatment for now.


Amethyst before heating


Amethyst after heating to 360 °C


Amethyst after heating to 385 °C
 

Six of the Best Specimens in the Mineral Kingdom!

March 25, 2021

By Peter Rosewarne

Introduction

Have you ever wondered if there was a specimen out there that was the world’s best, or what the best six or ten mineral specimens ever discovered are considered to be? I thought it might be a bit of fun to put together a “Six of the Best” of the mineral kingdom based on expert opinion in respected publications, such as The Mineral Record and its supplements, Masterpieces of the Mineral Kingdom, and American Mineral Treasures. Some of these discoveries g...


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THE MOST DIFFICULT JIGSAW PUZZLE OF ALL TIME

February 24, 2021
by
Duncan Miller

Imagine a jigsaw puzzle the size of the Earth, with most of the pieces missing. And those that aren’t missing are moving around all the time. This is the task that confronts some ambitious geologists. It is important because it explains why there are oceans and mountain chains, and why we may find rocks of similar ages and composition on far-flung continents. It also satisfies human scientific curiosity, and keeps some people employed and off the streets.

Until the mid-1960s...


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Botryoidal Minerals: A Colourful Trip Around the Mineral Kingdom

February 24, 2021

by
Peter Rosewarne

Introduction

My previous MinChat article on fluorite described a colourful trip around the world. In this article we take a colourful trip around the mineral kingdom, using minerals with a botryoidal habit as the guide. The idea came from the supplement to The Mineral Record of January-February 2020 on Mineral Collectors in Arizona, with the focus of one of the collectors being on botryoidal mineral specimens. The term botryoidal is derived from the Greek word botryios or ...


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FLUORITE - A COLOURFUL JOURNEY AROUND THE WORLD

January 15, 2021

by Peter Rosewarne

Fluorite: cubic, common, cheap (comparatively, but can be costly), contains calcium, and colourful, are some of the "C" words that can be used to describe this mineral. While good specimens of fluorite from classic localities aren’t cheap, most are cheaper than good specimens of ‘higher-end’ minerals such as azurite, dioptase, tourmaline and beryl and it is possible to build up a good collection of fluorites from worldwide localities. You are also likely to get a nice-...


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SAMS outing to the Springbok area, September 2020

December 4, 2020

Lesley Andrews


Jubilee copper slag dump, near Concordia

Towards the end of September, Richard and I met up with some members of SAMS (South African Micromount Society) in the Northern Cape. The Society is based in Gauteng, and the trip included site visits en route from Johannesburg. The Chairman of SAMS, Patrick Barrier, and Linda Stone, the President of FOSAGAMS, also joined the outing.

In the Northern Cape we stayed in accommodation at Springbok and Nababeep. This area is well-known for cop...


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Springbok Recce: Geology, Mining Heritage (and Wild Flowers)

December 4, 2020

By Peter Rosewarne


Figure 1 The Matzikammaberg at Vanrhynsdorp

This trip had its beginnings during the Lockdown with reading up on some books on South Africa’s mining heritage, geological sites and geological journeys. With the relaxation of travelling restrictions and reports of a bumper flower season in Namaqualand, I decided on the spur of the moment to do a trip to the Springbok area, which is rich in sites of geological and mining interest. My wife and I were going to go but, in the we...


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SOME UNUSUAL POLISHED SLABS

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 Me...


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PICKY PIGEONS PREFER POLISHED PEBBLES

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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DIGGING OUT DUMORTIERITE

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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