February 21, 2019

By Duncan Miller

(This is a follow-up to a previous article on faceting quartz, to be found with other faceting articles on the club’s website http://ctminsoc.org.za/articles/category/Faceting.) 

Every faceter knows quartz, those great big glassy-looking chunks that seem to cry out to be turned into doorknobs. Or pretty, golden ‘citrine’ that can cut brilliant yellow stones. Or glowing, dark purple amethyst with seductive blue flashes, dreamy rose quartz, or rutilated quartz with geometric golden blades. The range of possibilities is vast. The material is relatively inexpensive. What’s not to like? 

Quartz is not the easiest material to facet and often it is disheartening for the beginner battling to get a good polish. But let’s start at the beginning. If you are going to cut a lot of quartz you need a coarse lap to remove surplus material, or you need to grind preforms on a vertical wheel. A 320 mesh electro-bonded topper works well and when it wears out you just throw it away and buy another. Or you can charge a copper lap with coarse diamond yourself, and replenish the charge as necessary. Quartz swarf tends to clog diamond laps, so clean them thoroughly after use. I collect chunks of pumice off the local beach and grind that with lots of running water. It cleans electro-bonded and sintered bronze laps quickly and efficiently. Alternatively, scrub the lap under running water before the swarf has had time to dry out and set hard. 

Selecting material usually is easy. Unless it has attractive inclusions, and they cover a wide spectrum of the mineral world, avoid material with cracks and veils. Although, if you are lucky, internal flaws can produce rainbow interference colours that enhance an otherwise plain stone. Different colours of rough present their own selection issues. Amethyst can be too dark, resulting in an almost black stone. But many of these respond to cautious heat treatment in a closed kiln. Quartz from various different localities responds differently to treatment, either by heat and/or radiation, to produce a wide variety of colours not encountered naturally (see long discussion here: https://www.mindat.org/forum.php?read,55,350011,350166#msg-350166). 

The problems in cutting facets are few. The only difficulty I have experienced has been with some rutilated quartz, in which a few of the straight needles actually slid back and forth from one side of the stone to the other, protruding further and further as you cut opposing facets. The solution was a drop of cyanoacrylate (super glue) at either end to hold the needles in place during fine grinding. 

Polishing can be a different matter though, and many people struggle to get a good polish on quartz. As with all stones, a fine pre-polish – 3000 or 8000 mesh – is a pre-requisite. Cerium oxide and zirconium oxide are the oxides of choice for polishing quartz, using a very thin, water-based slurry. Polishing laps impregnated with these oxides are available. I have graduated from using a Lucite lap with cerium oxide to a Gearloose Creamway lap with zirconium oxide; and completely failed to be able to polish quartz with a Gearloose Darkside lap. Other people have no trouble with the Darkside. Some polish quartz with 60 000 mesh or 100 000 mesh diamond on soft metal laps like tin/lead or Batt, or on specially formulated polymer composite laps like Gearloose’s Diamatrix. You use whatever works for you.

One of the vexing issues in polishing quartz is caused by Brazil twinning and is particularly evident in amethyst polished with oxides. This takes the form of geometric pattterns of relief that appear on some facets, due to differential hardness in the twin lamellae. It is best avoided by ensuring that you have a very fine pre-polish and don’t need to polish for long, or to resort to diamond polishing.

 Photo: Geometric polishing relief caused by twinning in quartz, viewed with oblique lighting.

Below is a selection of different quartzes cut in various designs by Jo Wicht. Virtually no piece of rough used here cost more than R100, so quartz for her has proved a fun and inexpensive way of learning to cut gemstones as a hobby. Most of these stones were pre-polished at 3000#, and polished on a cerium oxide Ultralap. 

Varieties include: rock crystal, smoky quartz, rose quartz, green quartz (prasiolite), yellow quartz (citrine), purple quartz (amethyst), smoky quartz with rutile (largest sphere), blue chalcedony, blue lace agate, motorolite, chrysoprase, banded agate, and carnelian.

Photo and Cut Stones by Jo

105 ct Topaz in an Emerald Cut by Sigward Killat

This took a year to complete, partly because Sigward had to devise his own methods

 for polishing the very large table.



February 21, 2019

By Peter Rosewarne


Having built-up a mineral collection the question arises, at some stage, as to what to do with it looking to the future. Options include do nothing (and continue to get enjoyment out of looking at and handling the specimens) and let someone else worry about it when you’re gone (i.e. throw it away), give it away, donate it to an institution (probably unwise in SA or anywhere probably), or sell it. This article looks at some aspects of the pricing and selling process ba...

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January 19, 2019

By Duncan Miller

Topaz is a rather under-rated gemstone. This perhaps it due to the fact that pure, colourless topaz is relatively plentiful. Much of it is irradiated and then heat-treated to produce various intensities of bright blue. Natural blue topaz tends to be much paler, although dark blue stones do occur naturally. These are rare and hence more valuable. Natural topaz occurs in a wide variety of colours, including light green, yellow, orange and pink. The famous orangey-pink topaz fr...

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Two beached whales were spotted at Yzerfontein this past month, the first on 5th November and the second a few days later

November 23, 2018

And last week this fish appeared at Milnerton Lagoon

Grey chalcedony and aragonite fish

This is the same fish as above, but much prettier “when still alive”, and seen under short wave UV light.

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"There’s treasure – I just have to find it"

November 23, 2018

Talking about treasure hunting, let me tell you my story … It’s a tale of two parts.

I’ve always been the poster child for the story told by Victor Borge:  “if there’s manure, there must be a pony.”  It’s in my DNA.

Truth be told, my first real life encounter with this approach was doomed. 

To understand it better, you’d have to know that back then Dinner, Bed and Breakfast at a swanky hotel cost R40-00 and a full seafood buffet at the same hotel with all you could eat cos...

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FaceTips for December

November 23, 2018
by Duncan Miller

This month I will show you how to scale a GemCad diagram to a different L/W ratio. This is very easy if the diagram is a fully meetpoint diagram, without a preform. You note the initial L/W ratio from the Print Preview and then click on Scale in the Edit menu. Here you check the X box because you want to change the proportions in the X direction, then enter the appropriate numbers to divide by the initial L/W ratio and to multiply by the one you want, and press OK. The next me...

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FaceTips for November

November 4, 2018

By Duncan Miller

I started faceting in pre-GemCad days and found cutting ovals very laborious. I would cut the girdles by eye, using various oval templates, and placed the brilliant-style facets by eye too. Producing matching pairs was very trying. The advent of meetpoint faceting and GemCad overcame all these difficulties. Now there are lots of designs for ovals that are meetpoint, requiring no preform, with the girdle outline evolving out of the cutting sequence. You can access some of the...

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October 25, 2018

The Waldorf School asked us if they could visit the club again this year, and Claire Vaskys organised the day for them. Thank you very much Claire.

Also a big thank you to Rinda who had kept all the little offcuts of stones, and dopped them in preparation for the children to grind and polish, and who managed the workshop while they were busy between machines. 

Thank you to Marsiglio who brought his tools, raw and finished materials, and allowed the kids to take his rock pick and smash it in...

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Geological Tour of the Rosh Pinah area

September 25, 2018
Since we have never fully explored the southern parts of Namibia before, we decided to head up to Namibia a few days before the planned start of the FOSAGAMS Namibia 2018 tour to explore the area. Heidi Naudé from the Pretoria Club put us in touch with Gisela Hinder who owns the Rosh Pinah Geo Center and after some discussion with Gisela on our interests, we pre-booked a guided geology tour with her. Our first night in Namibia was spent camping along the Orange River, and a casual walk acros...
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FaceTips for October

September 25, 2018

By Duncan Miller

This was another jeweller’s request. The setter had broken one of a matching pair of blue-green stones, destined for earrings, bought by the client in India as emeralds. They were apatite; but nevertheless the broken stone had to be replaced to fit the already-made setting. Fortunately I had just one piece of blue-green apatite that matched the colour. In order to produce a stone of the same size and proportion I had to replicate the oval precisely. I could have slapped fa...

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