by
Duncan Miller

Inspired by an article by Pavel Martynov on mindat.org I have been experimenting with taking stereoscopic pair photographs of an otherwise rather uninteresting mineral specimen, so that it can be viewed in 3D. It is really simple. All you need do is rotate the specimen through five degrees, without otherwise changing its relationship to the camera, and taking two photographs of it. Then you swap the righthand photo for the left one, and view the pair by squinting. You can p...

by Duncan Miller

South Africa is famous for its gems and minerals, and visitors sometimes imagine they can pick diamonds off the soles of their shoes. Gold nuggets roll down streams and platinum can be picked out of some of the oldest rocks on Earth. This is all fantasy of course. The platinum, gold and diamond mines are so regulated that throughout their lives most South Africans never see native gold, or platinum, or an uncut diamond; and possessing them without a permit is a criminal offe...

Duncan Miller

There are several different mechanisms for garnets to show a change of colour. The most common of these is analogous to the colour change in the well-known alexandrite variety of chrysoberyl. This is due to differential transmission of different wavelengths of visible light, leading to a difference in perceived colour under lighting with different degrees of red or blue light. A less well-known type of colour change in gemstones is the so-called Usambara effect, named after t...

Lesley Andrews

Garnets are not only attractive, but also useful. As residents of the Cape, many of us know about the use of garnets as markers in diamond exploration. Other examples include garnet use in sandpaper, especially for wood sanding, and the production of synthetic garnets for laser generation.

The colours of garnet group minerals and varieties is a complex subject. Not all garnets are red, in fact they are found in all colours except bright blue. Briefly, the most common ions inf...

by
Peter Rosewarne

This month we are checking out the garnet group of minerals, which most people will have come across in the form of jewellery, as mineral specimens and even in mundane articles such as sandpaper. The previous fluorite article started with some “C” words that apply to the mineral and in the same vein, the following apply to garnets, with some qualifiers; cubic, contain calcium (some), colourful (some), cleavage-free, conchoidal fracture, costly (some), contain chrome ...

Jo Wicht

Who would have thought that some of the most boring looking mineral specimens could be the most spectacular under shortwave fluorescent light? Look at these three, for example…

 

 

 

Over time I have made a point of collecting specimens if I am aware that they fluoresce because that is fun, but often one acquires others unknowingly. Because I agreed to take some photos to supplement this edition of the MinChat, I shone my UV light over my entire mineral collection to see what I coul...

Duncan Miller

Cut rubies, red garnets and red glass can look very similar. There are several techniques that can be used to determine if a red stone is a ruby. These include a semi-destructive relative hardness test (ruby will scratch garnet and glass, but not the other way around); using a polariscope to test for birefringence (ruby is birefringent whereas glass and most garnet are not); and using a dichroscope to see the two pleochroic shades of red in ruby (which are absent in garnet and ...

Willie Lombard

I collect different species of minerals and rocks of SA and Namibia and need only one good example of each. Right across from the old Swanson Enterprises building in Springbok is an open yard with some large heaps of rocks and minerals. I asked the resident on the property if I could have a look-around. No problem. Found some fluorites and a good example of a diorite. There were some feldspars and a lot of pegmatites. I was sleeping over, so I asked the resident if I could ret...

Willie Lombard

On the shores of Lake Superior in the USA a fluorescent rock made headlines (on YouTube, anyway!). They call the normally drab rock a Yooper, after the locals from Upper Michigan. A geologist from the local university found that the sodalite in the rock causes the yellow fluorescence. I wish my sodalite would fluoresce like that! Those that do, produce only a very weak yellowish glow. On my way to the Groot Marico Gemboree in 2018 I decided to sleep over in the Pi...

By Peter Rosewarne

Introduction

The branch of mineralogy dealing with fluorescence apparently gained popularity in the 1930s with the availability of battery-powered portable ultraviolet (UV) lamps. The pioneer in producing such UV lamps and using them to prospect for and showcase minerals was Thomas S Warren, after whom the Thomas S Warren Museum of Fluorescence at Sterling Hill Mine Museum in the USA is named.

Those of you who have been paying attention to previous MinChat articles will ...

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...

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-...

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...

Duncan Miller

Peter Rosewarne

I’ve borrowed the title of a new coffee table book by Gerhard Wagner for this article as it seems appropriate in that the Tourmaline Group encompasses some 14 species currently and it is found in classic localities around the world. The idea for doing this article came from a comment from Jo that EXCO had raised tourmaline as a possible discussion topic. I also have and have had quite a few tourmaline specimens in my collection over the years and have attempted to limit dis...

by Jo Wicht and Duncan Miller

For several decades small mines in southern Namibia have produced an attractive banded agate marketed as lapidary material. The major source has been a mine on Ysterputs farm, producing blue lace agate. It was promoted widely by the late George Swanson who owned the mine, so this material with its wavy blue and white lines is quite familiar. What is less well known is that the blue lace agate from Ysterputs is accompanied by several minerals forming aesthetic, c...

by Peter Rosewarne

 About a year ago I wrote an article on selling a mineral collection from South Africa, which was featured in the MinChat. One of the methods I listed, unsurprisingly, was setting up my own internet site. I didn’t go that route initially, relying on selling back specimens to dealers such as Hummingbird Minerals, John Betts Fine Minerals, Fabre Minerals and The Mineral Gallery, and some on Club Open Days. However, the former route seems to have run its course in terms of ...

By Peter Rosewarne