Articles

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

"Their dress consists of all kinds of beautifully prepared skins...gorgeously ornamented with copper beads...Their locks they thread with copper beads, covering their heads all over.  Around their necks they have chains, slung round them 15 or 16 times.  Many have round copper plates suspended from these chains.  On their arms they have chains of copper and iron beads which go round their bodies 30 or 40 times.  Their legs are encased in plaited skins, ornamented with beads......

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...