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