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 facets on it and hoped for the best, but being a precision cutter I made a diagram to guide me. To produce the correct girdle outline it required a preform, for which the rough was too shallow to cut to a point, so I built it up with low temperature green wax. Once the girdle was established, following the cutting pattern was relatively easy, although not the sort of thing a meetpoint faceter would relish doing.

I am going to use this example to demonstrate how to us Robert Strickland’s GemCad program to make a preform from any faceting diagram that may need one. First, you need a copy of GemCad, which you can download for the very modest fee of US$95 from www.gemcad.com. Next, you need a GemCad copy of the design for which you wish to make a preform. This presumes it doesn’t have a girdle preform already and isn’t a meetpoint design that generates the girdle outline as you cut the design (like the Omni oval posted last month). Then, in GemCad you delete all the facet tiers except for the girdle facets. This leaves you with a tall prism shape. Simply cut a series of facets at the girdle indexes to meet at a common point on top. There, you have your preform. What angles to use? I usually start with 40˚ for the steepest facets, the ones where the girdle is nearest the centre point, but once you have the preform you can tangent ratio the facets in GemCad to accommodate your rough and the design.

Next month I will describe how to produce preforms for ovals of any standard proportion from scratch.


Is the gemstone in the above picture aquamarine or glass?

Look for tell-tale air bubbles in the gem – very often, if glass, they are there.  JW


 

Southern African Lapidary Stones to watch out for

September 25, 2018

Verdite

Verdite is a fairly soft South African stone found in the Barberton area. It is often seen in African curio shops carved into animals. Its golden flecks distinguish it from buddstone which is a much harder metamorphosed chert. Even more distinctive is the “leopard rock” which is spotted serpentinite, also from that area. JW

 

Buddstone



Leopard rock

 

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FaceTips for September

August 24, 2018

By Duncan Miller

Here is a quick and easy oval with a standard 1:1.30 proportion. It has a fully conical pavilion, so you can spin a conical preform, stopping just short of producing a point. This means you don’t have to change angles and mast height when cutting the sixteen pavilion facets, which saves time and avoids mistakes. This is a fully meet-point design that doesn’t require a preform, so it would be good for a beginner’s first oval. It doesn’t work well in quartz or beryl, s...


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STAVOREN MICROMOUNT OUTING

August 24, 2018

Jako Schonken

I arrived at OR Tambo airport on a cold winter’s Thursday morning, hoping it wouldn’t rain. I hired a car and started driving in a northerly direction, following my GPS to Marble Hall - a place I have only heard about in the South African Micromount Society’s newsletters. I have been a member of the South African Micromount Society (SAMS) for more than three years, but have never been to their meetings or outings since they are based in Gauteng. Nor have I ever met any of...


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FaceTips for August

July 25, 2018

By Duncan Miller

Jewellers sometimes ask for the impossible, and it’s a challenge to try and oblige. This design was developed to cut the citrine for a dome-shaped ring. It had to be a ‘classical’ mixed cut with curved girdle lines to match the curve of the top of the ring. This design requires a preform to get the girdle facets the right size. The relative depth of the pavilion tiers affects the angles of the triangular corner facets, but these can b...


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Southern African Lapidary Stones to watch out for:

July 25, 2018


SODALITE – this is generally sourced from Swartbooisdrif, near the Kunene River in north-west Namibia. It has a Mohs hardness of between 5,5–6, is generally fine grained, and works very well.

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Encouraging the Curiosity of the Next Generation

July 25, 2018

Matt Lurie (5 years old), who lives in Johannesburg, was curious as to the make-up of an (amethyst) Spirit quartz crystal so he bought one for R10 in Johannesburg and de-jacketed/exfoliated/peeled it, to satisfy his curiosity! The images below were taken by his dad (Robert’s son) Dan Lurie.

   


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Zeitz Museum of Contemporary African Art

May 24, 2018
Once the grain silos of Cape Town Harbour

Built in 1923 the old silos in Cape Town Harbour were used for storing grain awaiting export, and were in operation until 1995. Today they house a modern art gallery. Thanks to the generous sponsorship of Jochen Zeitz, (former CEO of Puma) and the incredible architectural imagination of the Heatherwick Design Studio, the building is now a magnificent place to visit. The old round tubes of silo have been cut away in specific places to open up a central ...


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Faceting

May 7, 2018

Here are two stones that Duncan Miller cut recently from rough he bought two months ago at Open Day. The yellow citrine (from Johann de Jongh) has only 58 facets, is 22 mm wide and weighs 37 ct. The design is ‘Xephyr’ by Arya Akhavan (yes, with an ‘X’). The light green fluorite (rough from Rob Smith), is the first one he has ever cut. It has even fewer facets, only 36, is 15,5 mm wide and weighs 16,2 ct. The design is slightly modified from ‘Six Shooter’ by the late Jeff Graham.  ...


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Visit to Mineral Services and the Exhibition “Messengers From The Mantle”

March 29, 2018


By kind invitation of Professor John Gurney, 25 club members visited Mineral Services on 24th February to see his exceptional exhibition “Messengers from the Mantle”. This irreplaceable collection of kimberlites was initially created for the 35th International Geological Congress at the Cape Town International Convention Centre in September 2016, and was displayed again at the 11th International Kimberlite Conference in Gaborone, Botswana in Septembe...


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