Synthetics

June 27, 2017

Duncan Miller

Synthetics are a wonderful source of relatively inexpensive faceting rough, in a wide array of colours, some of them not available at all in natural stones. On the whole, synthetic gem rough is predictable in its behaviour and also enables the cutter to explore quirky cuts in larger sizes than would be affordable in natural rough. And increasingly jewellers are setting well-cut synthetics in precious metal jewellery. So dive in, and enjoy yourself.

The most commonly available synthetics are cubic zirconia, corundum and spinel. Cubic zirconia is produced in angular chunks, often elongated, that look somewhat like natural crystals, but they are produced in huge quantities in industrial-scale furnaces. Early production of cubic zirconia was very expensive and coloured rough quite rare, but now it is readily available in a huge range of colours, some even displaying colour-change effects in different lighting. It is also obtainable in very large pieces now, allowing you to cut huge gemstones if you like punishment. Synthetic corundum also is produced in a vast range of colours, although the most commonly encountered colours mimic natural ruby and blue sapphire. The rough most commonly available is in split boules – finger-like single crystals split down the middle to relieve the built-in strain due to their manufacture. Most synthetic spinel is not an exact mineralogical copy of any natural spinel, but is structurally stabilised (as is cubic zirconia) by various additives. The boules of synthetic spinel are not split like corundum, but often have a characteristic network of surface cracks. There are numerous other synthetics available from specialist outlets (for example http://morioncompany.com/ or http://www.gemshub.com/rough-stones.php). These include quartz, beryl, YAG, and various diamond simulants.

The ‘common’ synthetics – cubic zirconia, corundum and spinel – are best treated like natural sapphire when cutting and polishing. This means facet cutting on standard mesh size diamond laps, either electrobonded, sintered, or rechargeable copper with loose diamond or paste. Corundum tends to produce an ‘orange peel’ surface on 1200 electrobonded or sintered diamond laps, so many people go straight from facet cutting with 600 mesh to fine cutting and pre-polishing with 3000 mesh loose diamond or paste on copper or zinc laps. After a good pre-polish, polishing the hard synthetics usually is not a problem with 50 000 mesh or 100 000 mesh diamond on tin/lead, Batt™, BA5T™ or Diamatrix™ laps (the last three available from Gearloose Lapidary). Some cubic zirconia, especially early production rough, can spall and cause scratching problems when polishing. It is cheap enough these day to discard troublesome rough.

Cubic zirconia and synthetic spinel are cubic, or pseudo-cubic, in structure and have no cleavage, so they do not have to be orientated crystallographically. The cracked ‘skin’ on synthetic spinel needs to be removed by preforming before facet cutting. The synthetic corundum is trigonal, with the c-axis lying nearly parallel to the split boule face. This means that in the ruby version the best colour is obtained with the table of the stone perpendicular to this face. Placing the table parallel to the split boule surface, which is tempting, produces a more unnatural looking colour. The deep blue boules that mimic natural blue sapphire often have a dark rind and a nearly colourless interior. It is important to exploit this dark rind to obtain a richly coloured stone. It is best to orientate the rind in the table if possible. Because of the curvature of the outside of the boule, on larger stones this often is not possible. The alternative placement of the blue rind in the culet can produce a stone with a darker centre and lighter periphery. More expensive blue corundum rough is available with the colour uniformly present throughout the stone.

One of the most worthwhile synthetics to cut is synthetic emerald. It is available in various grades with different densities of inclusions, to mimic natural material more closely. Large, perfectly clean synthetic emeralds tend to look a bit glassy and ‘artificial’, so a scattering of inclusions can make for a more realistic-looking stone. I am not advocating fraud here, and any synthetic must be declared as such for a legal sale.

A selection of cubic zirconia rough, showing some of the wide range of available colours (www.tradeindia.com)

A small selection of synthetic corundum, showing the typical size of the readily available split boules, about 12 mm in diameter

 

A selection of synthetic spinel boules, showing the range of colours from colourless to a deep ‘sapphire’ blue

Synthetic spinel boules, showing the typical network of surface cracks in the yellow boule

 

New Barite Occurrence at Rosh Pinah Mine – Namibia

May 23, 2017

Transparent to whitish barite mineralization was found in an orogenic late phase leached fault zone. It seems that some of the barites are pseudomorphs replaced by snow white baritocalcite. This replacement supposedly took place at an even later phase when calcium-rich fluids migrated through the formation. This theory is supported by the occurrence of floater quartz crystals in a pocket where, on the one side, the quartz aggregates display the luster of ‘bergkristall’ and are coated on t...


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The History behind the Mike Lurie Collection

May 23, 2017

The Lurie family lived in Bulawayo, Rhodesia/Zimbabwe, from 1951 to 1966. During this time Robert’s late father, Mike, worked as a manufacturer’s representative. His job took him by car all over Southern Rhodesia (Zimbabwe), Northern Rhodesia (Zambia) and Nyasaland (Malawi). Every now and again Mike would stop his car in the middle of the bush to take a break from the difficult, long distance driving. He would often notice something shining, or an agat...


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Working With Diamond

May 23, 2017

Duncan Miller

No, this is not about polishing diamonds, which in South Africa is illegal without a license, but about working with diamond grit or paste. For the coloured stone gem cutter, diamond paste is easier to source and to use. Loose grit and pastes are available in a range of mesh sizes, with crushed natural diamond or synthetic diamond. Synthetic diamond is made as single crystals and polycrystalline aggregates. The polycrystalline diamond breaks down with use to produce finer parti...


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TOURMALINE

April 25, 2017

Duncan Miller

Tourmaline can be temperamental. Rough tourmaline occurs in two distinct shapes – globular nodules and elongated pencil-like crystals elongated in the direction of the c-axis. The globular nodules sometimes spall concentrically, like onions, and the pencils sometime fracture transversely. This behaviour is difficult, if not impossible to predict, although fine cracks in the ‘skin’ of tourmaline pencils is not a good sign. The cracked skin must be removed by preforming or th...


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New Barite Occurrence at Rosh Pinah Mine – Namibia

March 25, 2017

Author: Gisela Hinder, Rosh Pinah Geo Center, Rosh Pinah, Namibia

e-mail: gisela.hinder007@gmail.com

 Rosh Pinah Mine is situated in the southwest of Namibia about 80 km east of Oranjemund. Rosh Pinah Mine became well known for its beautiful barites when a massive pocket of yellow to orange barite crystals was opened in 1989. It is said that these barites were the best ever found in Namibia.

In February 2017 new barite crystals were discovered at Rosh Pinah. Yellowish, unfortunately smallish, ...


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GARNET

March 25, 2017

Duncan Miller

Garnets are among the easiest gem materials to facet. They have no distinct cleavage, although some crystals have a parting that causes them to fracture into thin slabs. The rough often is in globular shapes, which is good for weight recovery. When choosing rough, avoid being fooled by fake material. Red glass is sometimes covered in adhering deceptive ‘grit’ to mimic natural nodules. Illuminated from behind or the side with a torch, the characteristic internal swirls and r...


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A Bit about Blue Lace Chalcedony

March 25, 2017

Jo Wicht

Blue Lace “Agate” is found on the farm Ysterputs 254 (meaning iron holes) in Namibia. The mine is located adjacent to the “Blinkpan” (shining shallow lake) which can be seen to the west of the B1 highway about 80 km north of Vioolsdrift and Noordoewer, which are the border towns on either side of the Orange River between South African and Namibia.

Blue lace is not a true agate, but a chalcedony (microcrystalline quartz) laid down in a series of wavy bands, which gives it th...


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The Beryl Family

February 24, 2017

Duncan Miller

Many faceters recommend that beginners start with aquamarine. It usually presents no problems in faceting or polishing, is relatively easy to obtain, and in lighter colour it is not overwhelmingly expensive. Aquamarine is the blue or blue-green gem variety of the mineral beryl, an aluminium beryllium silicate. It occurs in elongated hexagonal barrel-shaped crystals. It is dichroic, with the most intense colour when viewed along the length, the so-called c-axis. This is a pity, ...


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Faceting and Polishing Quartz

January 15, 2017

Duncan Miller

This is the first of an intended series of articles on faceting and polishing a variety of gemstones. I am beginning with quartz because that is what most people start faceting when they first take up the hobby. Quartz rough is inexpensive and readily available in a wide range of colours. It is not necessarily the easiest material to polish, but if a particular stone behaves badly it is no great loss to set it aside to be tackled at a later date. You should try to select rough ...


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