Coloured Gemstone Cutting
Coloured gemstone cutting is one of the oldest art forms
with records dating back to before the birth of Christ. It
has essentially remained unchanged for centuries.
Originally, gemstones used to be decorated by scratching
figures, symbols or letters onto them. This in turn lead to
the art of "Engraving".
The origins of gemstone cutting can be found in India. Up
until 1400 AD, there was very limited polishing done. This
was mainly to improve lustre and remove unsightly blemishes.
Idar-Oberstein became the centre of Agate and Coloured Stone
cutting in the 16th Century. Nowadays, in order to promote
and encourage local cutting centres, many governments have
started to restrict the export of rough materials. The major
cutting centres today are Sri Lanka, Thailand, India, Brazil
and Idar-Oberstein.
The manufacture of gemstones can be divided into 4 distinct
areas:
- Engraved Gemstones (i.e Cameos)
- Agates.
- Coloured Gemstones.
- Diamonds.
The aim of the cutter is to try and retain as much weight
as possible whilst ensuring that the most attractive colour
is visible through the table facet. It is also critical that
the cutter adhere to certain angles and proportions in order
to maximize overall brilliance in the gemstone.
Engraving
Refers to Cameos, Intaglios, objets d'art and other ornamental
pieces.
The oldest stone engraving consisted of cylinders that were
engraved with symbols and figures and used as Seals or Amulets.
Today the main centre is Idar-Oberstein where 90% of the
worlds gems are engraved.
Coloured Gemstones
Refers to all Coloured Gemstones except Diamonds.
Cutting gemstones is called lapidary work, with the cutter
referred to as a "Lapidary".
Most cutters tend to specialize in certain stones or a group
of stones.
Opaque Gemstones
Circular saws coated with diamond dust are used to first
of all cut the rough into manageable pieces or by utilizing
cleavage planes, which are directions of weakness in certain
gemstones, with a small hammer.
Soapy water, oil or paraffin are used as a coolant. Final
shaping usually occurs on a vertical roughly grained carborundum
wheel cooled with water.
The preferred style of cut for opaque gemstones, heavily
included stones or stones displaying "Optical Phenomena" is
the "Cabochon" cut.
Transparent Gemstones
The final shaping is done on a horizontal grinding wheel.
The stones are cemented into a 4-6 inch holder called a "Dop"
using a special cement or shellac. Nowadays, special clamps
are used as well. "Dops" are guided at an angle related to
the facet being cut.
This is achieved either by inserting the wooden "Dop" into
pre-drilled holes set at certain angles or by more modern
devices used to control and ensure the proper angles are used.
Cutting wheels are usually made of lead, bronze, copper or
tin. Polishing powders usually consist of Carborundum,
Diamond, Titanium or Carbide. The speed of the wheel, type
of wheel and type of powder used all vary with the stone being
cut.
The final polishing takes place on a horizontal wheel, wooden
cylinder or leather straps to remove the final traces of scratches
and also to improve the lustre.
Polishing compounds used are all finely grained and consist
of Tripoli, Chromium Oxide, Diamond Powder, Garnet, Emery
or Iron Oxide mixed with either water or in some cases Sulphuric
or Acetic Acid.
Factors to be Considered When Cutting Coloured Gemstones
The cutting of small gemstones is only viable if the labour
is inexpensive and in plentiful supply.
Automated faceting machines have recently been introduced
but at present have limited applications.
The Lapidary must consider a number of variables before cutting
a gemstone. These include:-
- The shape of the rough and how it can be utilized to maximize
yield.
- How to ensure that the most attractive colour is visible
when the stone is viewed in a face up position.
- The position of fractures or inclusions which could affect
the clarity of the stone.
- The correct angles and proportions which will be needed
to maximize brilliance.
- How to juggle these factors to maximize profits.
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