Cutting and Setting

Gems are cut to reveal their beauty, made to get the best of the stone which isn't visible when freshly mined. Creates value from the rough. Some gems may produce several gems or stay as one - smaller stones may be better to make the best of the colour when cut a certain way.

Main consideration when cutting is a balance between beauty and the weight. What is the best yield you can get to produce the most beautiful stone? What is the best for profitability?

Types of cuts - common face-up shapes are pear, round, rectangle, square, oval; cutting-style inc. cabochon or faceted (to reveal brilliance, lustre, colour). Can change face-up width eg: smaller to get more brilliance, fire or lustre, facet arrangement or depth of stone.

History

In antiquity was probably just polishing fragments in a rounded style (beads and carvings) but through time and improved technology we learned to use harder materials to cut less hard materials. So started to fashion gemstones from c. mid 1400s.

In past gems up to Mohs' 7 could be drilled and ground to shapes using commonly found materials eg: sand as an abrasive, flint to scratch. Typical of jewellery in Ancient Egypt with gems of lapis lazuli, turquoise and quartz (carnelian and onyx).

Advancement in techniques allowed to work on harder gems. 3000 yrs ago used 'emery', a gritty sapphire to cut and polish gems like garnet, emerald and sapphire and corundum was used to polish garnet cabochons. 1800 yrs ago Roman empire, many transparent gems were being worked and set in jewellery - sapp., emerald, garnet, aquam., amethyst, citrine. Chips of diamond were used to drill other gems but itself was not cut until medieval times.

Use of diamond grit, along with invention of cutting tools and powered machines enable the cutting of all gemstones. Diamond was traditionally cut using abrasive-fed rotating plates called wheels. Lasers since introduced and manual labour increasingly replaced by computer-controlled machines.

Now, fine stones still modified by skilled cutters with experience and knowledge. Some exceptional cut stones may pass to an expert cutter to be evaluated and re-cut to bring new sparkle.

Cutting

Cutters look to make best of the stone in terms of maximising color & liveliness and minimising flaws and getting max yield re carat weight, compared to the original rough. Heavier the gemstone the more valuable generally. May maximise the weight but if there is an inclusion may want to cut it away. Would it be worth more to be a smaller stone of better clarity? There are also fashionable shapes, sizes and colours. Generally pay more for matching pairs of gems.

Colour- where colour is uneven, certain orientations and shapes will give a finer stone, even if not optimum size. In pleochroistic stones orientation is important to get colour so size may be sacrificed for colour eg: Tanzanite, where best blue is often orientated in a face-up direction.

Planning - on receiving a rough, cutters/lapidaries make a decision on cutting. Decisions are made on assessing return on investment, balancing visual appearance and it's final weight. Produce a simple, polished bead or a complex faceted stone? Today can scan the rough & use computer software & instruments to design how to cut. This is done particularly with diamonds. Once the final gem form is decided a rough is blocked to give basic shape, ie: face-up outline, then cut and polished.

Facets - flat, polished surfaces/plane on gem material. Added by humans to reveal beauty [on crystals there are flat, planar faces, arranged by the underlying crystal structure and naturally formed]. Used mainly for transp., coloured & colourless stones.

Cabochon - non-faceted consisting of domes, polished surface, base flat, convex or hollowed.

Flat shapes reflect and refract light to create brilliance and sparkle and usually reserved for transp. gems.

Faceting shapes.

Faceting terms.

Need to understand the nature of the gem with its optical properties and pleochroism. Cutters consider faceting in terms of shape, proportion and symmetry. Involves optical performance factors of brilliance, extinction & windowing.

Brilliance:

• light return, amount of light sent back or returned from the surface lustre and interior of a faceted stone in the face-up position.
• affects how colour is perceived
• high brilliance means good light return

Extinction:

• relates to dark areas of a faceted stone when seen from the face-up position
• if light leaks out the back of a stone it will have a darker, overall look and appear darker than it is

Windowing:

• see-through effect
• when we can see what is behind the stone
• light enters from the table facet, reaches the pavilion (back) and instead of being reflected back in some areas it passes through.
• in these cases brill is low and stone does not appear very ‘lively’, appears dull at the centre with colour only at the rim and edges

Other forms are cabochon / beads / tumble polishing / engraving.

Cabochon:

• one of the oldest and simplest forms, still common
• for gems where little, if any, light passes through
• makes the most of the colour or optical effects where the dome shape allow them to be seen
• French word from Latin meaning ‘head’
• good for red carnelian or lapis lazuli for colour, or rubellite which may lack clarity and the cabochon minimised this

Beads:

• many materials and shapes, very popular
• drill a hole to make wearable, then shape and polish
• recently have faceted beads for transparent material

Tumble polishing:

• when gems are washed downstream they are rolled and abraded to become smooth and tumble polishing recreates this
• often quartz varieties, rolled in a rotating drum with water, where they rub against each other to become smooth – coarse abrasives are used first then replaced with finer – they become rounded and irregular in shape
• common for agates & other opaque stones
• vibrating tumblers use vibration not rotation
• soft stones or those with hardness much over quartz can’t be tumbled successfully
• used for beads, to be set in jewellery or collected

Engraving:

• ancient to practice to make a seal by engraving
• uses a small, rotating drill & abrasive or scratched or gouging techniques
• today – laser cutting & ultrasound abrasion
• gems with engraved designs & inscriptions = intaglio (in – carved into)
• gems carved with design in relief (raised above surrounding surface) = cameo. Often use agate due to its colour banding, or shell

Modern cutting is an art and a science. Recently has been more experimentation with concave facets. Can be used to increase brilliance as light reflects at many angles. Hard to do so more expensive than trad cuts. High value gem material reserved for flat facets but cheaper material (eg: amethyst or citrine) can look wonderful with concave facets.

Fantasy cuts:

• Berndt Munsteiner, 3rd gem cutter generation from Idar-Oberstein, Germany, experimented in 1960s with ‘free cutting’ now known as fantasy
• included concave cuts on backs of stones
• style is now popular and prompted the ‘Cutting Edge Awards’ held by American Gem Trade Ass were lapidaries submit fantasy cut gemstones
• usually larger roughs as need more material to shape

Setting

Earliest held in place with adhesive or tying beads with wire or thread. Most gems now secured by a form of metal – folded, pushed over edges to keep gem in place. Gems also held in place with prongs/claws bent over the gem. Until recently most transparent gems continued to have closed backed by metallic foil or coloured pigments to maximize colour & sparkle. Not considered fraud but made stone look better.

As cutting improved, open-backed setting come into fashion. Discovery of new sources of fine gems, early 18th century on, eg: Brazil for diamonds, amethyst and topaz, increased this trend. Then closed backed and foils were seldom used for anything other than glass imitation gems. 1700s fine coloured gems became popular & prompted market for good imitations. Improved glass, greater brilliance, was introduced. Foil backing continued to be used.

Platinum, then white gold, came into fashion early 1900s and gave a new background for coloured gems. Platinum has strength to allow more delicate constructions. Setting techniques continue to develop and now some gems are cast in place in the jewellery as it’s made.

Two reasons why foiling of gems as become less common 1) better cutting 2) greater availability of gems.