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 menu offers to round the indexes to the nearest notches. As we are working with a 96 index wheel it will use that. Press YES – horrors, see all those factional indexes! – then press OK, and they will vanish.

OK, now let’s do it with an example. I am going to use the same mixed oval 1:1.30 design as last month. Here is a screen shot of the design.



Here is the Edit Scale menu box with the appropriate entries.



And here is the index rounding option.

Pressing YES produces a terrible list of fractional indexes, but OK in the next menu will round them. The end result looks like this.



If there are grey cut-offs because the diagram now is too long, you can Scale All to shrink it a bit for the diagram to fit. (I did that here.) The preform doesn’t rescale, so it is meaningless. You can remove it with Preform Delete. If you are going to save the rescaled diagram you should change the heading using Edit Heading/Footnote. The final diagram is below.




There are several things to notice about this rescaling. Crown star facets 9 will need tweaking to meet. The girdle indexes have changed from the 02, 08, 13, 21 set to 02, 06, 11, 19. This is the same as given for a L/W ratio of 1.50 in Table D-5 of Long & Steele, in the table below. This means we can use the Long & Steele recommended preform angles for L/W 1.50 to cut a preform for this design. And remember that in GemCad you can change the pavilion culet angles easily to deepen the stone if necessary, using the New angle option in the Facet dialog box that appears if you click on a facet.

Extract of Table D-5 SIXTEEN FACET GIRDLE OUTLINES FOR SELECTED OVALS (from Long & Steele, Facet Design Volume 1 Ovals, second edition)

L/W ratio

Indicies (96 wheel)

Angle (degrees)

Typical size (mm)

1.20

02, 08, 13, 21, etc.

45.0; 43.5; 42.1; 40.1

5×6, 10×12, 15×18

1.22

02, 08, 13, 21, etc.

45.0; 43.4; 41.8; 39.6

9×11, 18×22, 27×33

1.25

02, 08, 13, 21, etc.

45.0; 43.1; 41.4; 39.0

4×5, 8×10, 12×15, 16×20

1.29

02, 07, 12, 20, etc.

45.0; 43.3; 41.3; 38.6

7×9, 14×18, 21×27

1.33

02, 07, 12, 20, etc.

45.0; 43.0; 40.6; 37.6

3×4, 6×8, 9×12, 12×16

1.40

02, 07, 12, 20, etc.

45.0; 42.6; 39.7; 36.2

5×7, 10×14, 15×21

1.50

02, 06, 11, 19, etc.

45.0; 42.7; 39.2; 34.9

2×3, 4×6, 6×9, 8×12

 Scaling other designs that have preforms means you will have to construct a new preform from the rescaled design, using the technique described in the MinChat of October 2018.

 

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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SEPTEMBER VISIT FROM CONSTANTIA WALDORF SCHOOL

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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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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