When light passes through a doubly refractive gemstone, the light is split into two rays which are polarized at right angles to each other and travel at differing velocities through the gemstone. In some coloured doubly refractive gemstones, these rays may emerge differing in shade or colour. When this occurs, the rays are said to have experienced "Differential Selective Absorption".
Differential Selective Absorption:
Variations in the absorption of certain wavelengths dependent on direction, causing a stone to appear differently coloured in different directions.
D.S.A in :
Singly refractive Gemstones: No D.S.A, stone is the same colour in all directions. Monochroic.
Doubly refractive Gemstones:
Uniaxial - Two colours seen. Dichroic.
Biaxial - Three colours seen. Trichroic
Another term used for Differential Selective Absorption is Pleochroism.
Dichroscope
An instrument comprising of a suitably cut rhomb of Iceland Spar (Calcite) and a lens system in a short tube with a small square aperture at the other end.
The dichroscope separates the polarized rays so that they may be observed side by side.
Procedure:
- ace the gemstone on the rotating platform of the dichroscope holder table down.
- Direct light onto the gemstone.
- View the stone through the dichroscope.
- Slowly turn the rotating stage so as to view to all directions.
- Look for different colours or shades of colour.
Factors:
- Dichroism is only seen in "Doubly Refractive" gemstones.
- It is not seen in gemstones that display "Anomalous Birefringence".
- May be very weak or even undetectable.
- Not seen in colourless doubly refractive gemstones or along an optic axis.
- Exact colours are not important.
- Helpful in distinguishing:
- Blue Sapphire from Synthetic or Natural Blue Spinel.
- Ruby from Garnet or Spinel.
- Emerald from Demantoid or Tsavorite Garnet.
- Any doubly refractive gemstone (ie Amethyst) from Glass.
Dichroism can also be used in colour grading coloured gemstones.
i.e Blue Sapphire described as Greenish/BLUE
Dichroism can also be detected by using a polarizing filter. If it is rotated against a gem, first one colour will be seen (caused by one polarized ray) and then another. It can also be seen simultaneously by using a piece of suitably cut polaroid.
Luminescence
The effect noticed in some substances of giving out visible light when they are rubbed or scratched (Triboluminescence) or when they are irradiated with invisible electro-magnetic radiations (Fluorescence, Phosphorescence or Thermoluminescence).
| Fluorescence | produced by exposure to invisible radiations, such as ultra-violet light or x-rays. |
| Phosphorescence | the afterglow of fluorescence when the stimulating radiations have been shut off. |
| Thermoluminescence | a secondary light generated by certain substances when heated with invisible infra-red rays. |
| Triboluminescence | the phenomenon exhibited by certain minerals when they are rubbed or scratched of exhibiting luminosity. |
| Cathodoluminescence | fluorescent effect displayed by some materials when they are bombarded with a been of electrons |
| Electroluminescence | form of luminescence produced by passing an electric current through them. Natural blue diamonds can be separated from artificially coloured blue diamonds using this technique since the former are semi-conductors while the latter are not. |
Two methods used in Gemmology to produce luminescence are:
- Ultra-violet Light
- X-Rays
Ultra-violet Light:
The invisible rays, in the wavelength beyond the visible violet, having a range of between 100 and 380 nanometres.
| Longwave | 380 to 300 nanometres |
| Shortwave | 300 to 200 nanometres |
Fluorescence
| | | |
| Paste (Glass) | Inert | Bright pale blue or green |
| Natural Blue Sapphire | Inert (due to Iron) | Inert |
| Synthetic Blue Sapphire | Inert | Greenish-blue due to Titanium |
| Natural Blue Spinel | Inert | Inert |
| Synthetic Blue Spinel | Red (due to cobalt) | Bluish-white |
| Cubic Zirconia | Mustard yellow | Less pronounced |
| Natural Purple Sapphire | Inert | Inert |
| Synthetic Purple Sapphire | Red | Bluish-white |
| Natural Ruby | Red | Red |
| Synthetic Ruby | More intense red | More intense red |
| Garnet | Inert | Inert |
| Natural Emerald | Green to Red | Inert |
| Synthetic Emerald | Strong red glow | Strong red |
| Diamond | Sky-blue, blue, green to yellow. Stones that fluoresce blue have a yellow afterglow | less pronounced |
X-Rays
Electro-magnetic radiations of extremely short wavelengths falling between the ultra-violet and gamma rays of the electro-magnetic spectrum. Effects seen under X-rays are less variable than those under U.V light and are therefore considered more conclusive.
Uses:
- Used in the separation of Diamonds from its host rock. All diamonds fluoresce a chalky-blue when exposed to X-Rays.
- Used to distinguish Natural from Cultured Pearls.
- Used to separate awkward Synthetic Rubies from Natural Rubies. Most Synthetic Rubies phosphoresce under X-Rays while Naturals do not.
Chelsea Filter
Developed jointly in the 1934 by the Gem Testing Laboratory of the London Chamber of Commerce and the Chelsea College of Science and Technology, the Chelsea Filter was primarily used to distinguish Emeralds from its many simulants.
The filter consists of a combination of two gelatine filters that transmit only deep red and yellow/green light. This combination was chosen since Emeralds transmit light in the deep red but absorb light in the yellow/green.
The best results are obtained when stones are examined under a strong electric light (not fluorescent light). By holding the filter close to the eye with the stone(s) receiving as much light as possible, the following reactions can be observed:
GREEN STONES
| Natural Emeralds | Usually appear pinkish or reddish since Emeralds absorb in the yellow/green |
| Synthetic Emeralds | Similar reaction but in most cases they appear a more intense red |
| Emerald Simulants | Pastes (Glass), Soude Emeralds and Doublets appear green |
| Demantoid Garnet & Green Zircon | Similar to Emerald but will produce a negative reading on the refractometer (the scale will appear uniformly dark since no total internal reflection occurs) |
| Green Tourmaline | Green |
| Chrome Green Tourmaline | Red or Pink |
| Synthetic Green Spinel | Green |
| Peridot & Green Sapphire | Green |
BLUE STONES
| Synthetic Dark Blue Spinel or Natural Cobalt Blue Spinel | Red |
| Synthetic Light Blue Spinel | Orange |
| Zircon | Green |
| Cobalt Glass | Deep red |
| Aquamarine & Blue Topaz | Green |
| Sapphire | Dark green, almost black |
| Lapis Lazuli | Weak brownish-red |
Exceptions:
Not all Emeralds will appear pinkish or reddish through the filter.
Blue Sapphires which show a purple colour change when viewed in artificial light usually appear red.
Natural Blue Spinels appear red.
OTHER USES:
The Chelsea Filter can also be used to separate Natural Green Jadeite from colour enhanced Jadeite.
If the stone appears red under the filter this is positive proof that the stone has been treated.
If the stone appears green, further tests should be carried out since some colour enhanced Jadeite remain inert under the filter.
Microscopic Analysis
The best way to examine a gemstone is by using a microscope.
The microscope provides better magnification, illumination and mechanical stability. It consists of a set of eyepieces, viewing tube, objectives, coarse and fine adjustments, stage with built in illumination (i.e Reflected light / Darkfield illumination). It is an essential tool for the gemmologist especially in the identification of synthetics.
Gemmological Applications:
- Detection of synthetics and imitations.
- Study inclusions to assist in determining identity or place of origin.
- Detect double refraction (i.e Zircon or Peridot)
- Detect composite or assembled stones.
- Diamond or Coloured Stone clarity grading.
- Proportion grading for both Diamonds and Coloured Stones.
- Becke line method of R.I. determination.
- Direct method of R.I. determination when fitted with a calibrated scale.
- Study interference figures to determine whether it is uniaxial or biaxial when fitted with a polarizer/analyzer.
Thermal Conductivity
The Diamond Tester
Diamond has a much higher thermal conductivity than any of its simulants, and the tester uses this property to identify both mounted and unmounted stones. The instrument is particularly suited to testing small stones that are recessed in their mount.
Consists of:
- Hand-held probe that is fitted with a miniature heating sensor.
- A control box containing the associated electronic sampling and indicating circuits.
Procedure:
- Short pulses of heat are fed to the probe's silver test tip.
- The electronic circuits measure the rate at which this is conducted away into the gemstone under test.
- A high rate of heat transference will cause the green light to flash indicating that the stone being tested is a diamond.
- A low rate of heat transference will cause the red light to flash indicating that the stone is probably not a diamond.
Do's:
- Always check the tester against the test plate supplied or a known diamond before using it.
- Always allow the rings to cool before testing them if they have just been removed from the hand.
- Always test unmounted stones by placing them table down on a test plate and touching the probe to the culet.
- Always allow two to three second intervals between tests.
- Always check stones twice.
- Always return the probe to the holder after use.
- Allow all stones to stabilize to room temperature.
Don'ts:
- Do not hold the stone or the mount with your fingers.
- Never apply pressure to the probe.
- Do not allow the tip of the probe to touch the metal mount.
- Rely solely on the results of a diamond tester.
Specific Gravity
Definition:
The weight of a gemstone in air compared to an equal volume of pure water at 4 degrees Celsius.
1 cc of water at 4 degrees Celsius = One gram.Formula: Weight in Air
Weight in Air - Weight in Water
Methods for determining Specific Gravity- Displacement Method.
- Hydrostatic Method.
- Heavy Liquids.
Displacement Method
Suitable only for determining the Specific Gravity of large objects.
- Weigh the object in air.
- Fill a Eureka Can with water at 4 degrees Celsius until the water starts to flow from the spout.
- Allow the water to stabilize and the overflow to cease.
- Place a graduated vessel (marked in cc's) under the spout.
- Carefully lower the object to be tested into the water.
- Convert the water displaced (measured in cc's and representing the volume of the object) into grams.
- Use the S.G formula.
Hydrostatic Method
Procedure:
- Weigh the stone in air using a precision scale.
- Adapt a beam balance scale by placing a four legged stand over the left hand pan so that the swing is unaffected.
- Place on the stand a beaker of water at 4 degrees Celsius.
- Using non-capillary wire, make a cage and attach the wire to the weigh pan hanger so that the cage is immersed in the water and stays submerged during each full swing without touching the sides.
- Attach a counterpoise to the other weigh pan, shorter than the other side since it will have experienced weight loss by being submerged in the water.
- Weigh the stone in the water.
- Use the S.G Formula.
Heavy Liquids
Recommended liquids:
| | |
| Methylene Iodide | 3.32 |
| Bromoform | 2.89 |
| Monobromonapthalene | 1.49 |
| Benzyl Benzoate | 1.11 |
| Water | 1.00 |
Varying S.G's can be achieved by diluting Methylene Iodide or Bromoform with either Benzyl Benzoate or Monobromonapthalene.
Procedure:
- Clean the stone to be tested thoroughly.
- Place the stone in one of the Heavy Liquids.
- Observe one of the following reactions:
- If the stone floats - the stone has an S.G lower than the liquid.
- If the stone sinks - the stone has an S.G greater than the liquid.
- If the stone becomes freely suspended in the liquid - the S.G of the stone is equal to that of the liquid.
Disadvantages of S.G Determination
- The Hydrostatic Method is only good for larger stones, under 3.00 carats errors are evident.
- The Hydrostatic Method is time consuming.
- Heavy liquids can damage porous stones.
- Can only be used for unmounted stones.
- Heavy liquids are unpleasant to deal with and in some cases
- can be dangerous.
Advantages
- Useful when dealing with unpolished stones or stones of high Refractive Index.
- Easy to use (Heavy Liquids).
Amber Test
Procedure :
- Dissolve 10 teaspoons of salt into 1/2 pint of water producing an S.G of 1.12 to 1.14
Observations :
Amber and Copal Resin will float while all other Amber imitations will sink.
Gemstone Enhancements
The various enhancements used today include:-
- Heat Treatment.
- Irradiation.
- Impregnations.
- Surface Modifications.
- Composite or Assembled Stones.
Heat Treatment
Is defined as the controlled heating of certain stones in order to effect a change of colour. Certain stones are amenable to such heating and are considered quite stable while others are not. Since no two stones are alike, each possesses the potential to react differently. Stones in which the heat treatment is considered stable are considered commercially acceptable.
Detection
Detection for the most part is difficult. In some cases, identification is based on the fact that certain colours are rarely found in nature and are therefore considered to be enhanced. In other instances, the procedure is so widespread ( i.e Citrine Quartz) that it is automatically assumed to be treated.
Example: Blue Zircon which is produced by heat treating Brown Zircon.
Irradiation
Confined to a smaller group of gemstones, this process can occur in a variety of ways:-
- Radium Treatment
- Electro-magnetic bombardment
- Neutron Radiation
- Electron Radiation
Radium Treatment
Caused through exposure to alpha particles emitted by radium salts.
Detection:
Stones are radioactive and will fog a photographic plate if left in contact for several hours in a light proof box. The colouration (green in the case of diamond) is on the surface only and is normally detectable upon close examination.
Electro-Magnetic Bombardment
Bombardment by particles accelerated to enormous speeds in a cyclotron. In diamonds, the colour produced varies from green to black dependent on the length of the treatment.
Detection:
Detected by immersion in a highly refractive liquid since the colour is on the surface only. Yellow diamonds produced by further heating show a characteristic absorption band not seen in natural yellow diamonds. Treated stones will invariably show a ring around the girdle or an umbrella effect around the culet.
Neutron Radiation
Irradiation with neutrons from an atomic reactor. Colouration is through the entire stone rather than on the surface only.
Detection:
Brown and yellow diamonds produced by this method have a characteristic absorption band at 594nm but green diamonds are undetectable except for a variance in colour compared to the naturally occurring stone.
Electron Radiation
Colouration produced by using an accelerator. Colouration in the case of diamonds ranges from a pale blue to a blue green but is on the surface only. Colour resembles a rare type of diamond known as Type llb diamonds.
Detection:
Detectable by the fact that treated diamonds are non-conductors of electricity while natural blue diamonds are semi-conductors. Surface colouration is evident when the stones are immersed in a highly refractive liquid.
Most irradiated stones are considered stable but there are more cases of instability than in heat treated stones. Again commercial acceptance hinges on the degree of stability while public disclosure is open for debate.
Impregnations
These can include any of the following:-
- Bleaching
- Impregnation with colourless or coloured oils, waxes or plastics.
- General dyeing
Detection:
- In most cases, the stones are merely used to imitate more expensive stones ( i.e Chalcedony ) and can be identified by variances in physical properties and by microscopic examination.
- In the case of dyed green Jadeite, identification is possible by the variances in the absorption spectra and by its reddish or pinkish appearance under the Chelsea Filter.
- Some stones may fade under strong light.
- Immersion in an R.I liquid may reveal localization of colour. ( Should not be used for porous stones )
- Rubbing the material with a cloth or cotton swab previously soaked in acetone.
- Colour rubbing off on the thread used for beads.
Reasons for using impregnations
Colourless:
- Hide surface cracks.
- To stabilize material ( i.e Turquoise) or to protect it from acidic skin oils.
- Reduce porosity and improve colour.
- Produce a hard coating over a roughly ground surface to give the impression of a polished surface.
- Provide protection for softer stones.
Coloured:
- Fill cracks thereby improving colour.
- To imitate other stones.
Surface Modifications
Can be through the application of any of the following:-
- Wax
- Ink
- Paint or Varnish
- Foil backs
- Mirror backs
- Synthetic overgrowth.
Composite Stones
Stones constructed of two or more pieces of material which may or may not be genuine. Composite stones can fall into one of the following categories:-
- True Doublets - consisting of two pieces of genuine stone ( i.e Sapphire/Sapphire )
- False Doublets - where the pavilion is normally glass or an inferior stone.
- Triplets - consisting of three pieces.
Detection:
- Set in a liquid of similar refractive index. In the case of a Soude Emerald, the colourless spinel crown and pavilion will disappear in methylene iodide leaving a disc of green gelatine.
- Joining line visible under microscopic examination.
- Differences in lustre.
- Bubbles where the two planes meet.
- Refractometer tests are useful since most crowns are either quartz, spinel or glass.
- Red "Halo" effect seen around the girdle of an Almandine Garnet doublet ( in colours other than red ) when placed table down on a piece of white paper.
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.
Gemstone Deposits
A deposit is defined as a group of occurrences large enough to be worked (also called a mine). A find is described as a single occurrence.
Primary deposits refer to gems found in their original location. The yield is generally low due to the fact that many tons of non-gem bearing material have to be excavated.
Secondary deposits refer to gems which have been transported by the actions of wind, rain and flowing water.
Fluvial deposits are created by rivers, marine deposits by the sea and aeolian Deposits by the wind.
The distribution of gems around the world is irregular. Gem deposits occupy only a tiny proportion of the earth's crust. Some of the more favoured regions are:
The gem-rich metamorphosed limestones of Upper Mayanmar (including the "Mogok Stone Tract") renowned for Rubies, Sapphires and Spinels.
The alluvial deposits of Sri Lanka, Vietnam, Thailand and Madagascar and the Minas Gerais and Minas Novas districts of Brazil.Mining Methods
Most gemstones are discovered by accident. There is no systematic approach used for Coloured Gemstones due to the lack of capital. Exceptions being Australia where the mining of Sapphires is more mechanized and systematic.
Recovery is accomplished invariably by simple means without the use of modern techniques or scientific basis. In many cases, the methods are very primitive and have remained unchanged virtually for 2,000 years.
They consist of:-- Collecting gems from the surface, from dry river beds, or rock fissures.
- Sinking shafts into the ground sometimes up to 30ft deep.
- Panning rivers.
- Driving short tunnels into the sides of hillsides (used for mining Ruby, Sapphire in Mayanmar and Emerald in Colombia)
- Open-cast mining (i.e Ammolite)
- Using powerful jets of water, when available, to loosen the gem material from the overburden.
- Digging pits into ancient river beds to reach the "Gem Gravels", (i.e Sri Lanka, Mayanmar, Thailand).
- Terrace mining (i.e The Chivor Mine in Colombia).
- Underground mining which is expensive and can only be justified if a significant vein is located.
Gemstone Mining
Famous International Gem Mines
Sri Lanka
Sri Lanka is situated in the Indian Ocean south-west of India. It is bordered by the Indian Ocean to the north, south, east and west. It is located a few degrees north of the equator and is the 24th largest island covering some 25,332 square miles.
The population is approximately 14 million comprising of 79% Sinhalese, 13% Tamil (both South Indian & Sri Lankan Tamils) and 7% Moors.
Sri Lanka was formerly known as Ceylon until May 22nd, 1972.
Ceylon is the ancient Sinhalese name meaning "Resplendent Land",also known as SERENDIB or TRAPROBANE to ancient navigators.
The climate and vegetation are both tropical. Warm and humid at sea level (average temperatures 78 F to 82 F), the humidity during the day can be 70% (at night 90%). In the mountainous regions, average temperatures are 60 F. The monsoon seasons are from May through June and September through November. Average rainfall varies from below 40 inches in the dry northern zone and on the eastern slopes of the mountains to over 200 inches in some places on the south-western slopes. March to April are the hottest months.
Sri Lanka is an important source for not only gems but also tea, spices, rubber and coconuts. Approximately 2/3rds of the cultivated land and 2/3rds of the population are involved in the production and distribution of these products. Tea, rubber, coconuts and rice make up 95% of the countries exports and account for 75% of the cultivated land. Tea brings in over 66% of the yearly income from exports. In fact, Sri Lanka is the second largest producer of tea.
Rubber accounts for 15% of the yearly export income and coconuts account for 14%.
Gems continue to be a small and significant product of Sri
Lanka and has supplied the world with fine Rubies and Sapphires for over 2,000 years. Many consider Sri Lanka to be the original source of Ruby and Sapphire.
Although Burma has produced finer qualities, Sri Lanka has produced Rubies and Sapphires in larger sizes for longer periods of time.
All varieties of Corundum are found in Sri Lanka including the "PADPARADSCHA" which is unique to the island.
Main Gem Centres of Sri Lanka- Ratnapura : The oldest known source in Sri Lanka, meaning "City of Rubies" or "City of Gems".
- Elehara District : important in recent years for producing large and fine specimens of many gems including Sapphire and Cat's Eye Chrysoberyl.
- Area around Morawaka : known for Alexandrite and Cat's Eye Chrysoberyl.
- Nuwara Eliya : situated in the mountainous tea country.
Mining
Mining in Sri Lanka is mainly alluvial with gem deposits found not only in present river systems but also below rice paddies where ancient rivers once flowed.
The depth of the gem gravel or "ILLAM" varies from 3m to 20m to as much as 40m at Pelmadulla.
The search for gems is a highly speculative operation and is usually carried out by a group of native workmen on a share basis. 1/5th goes to the owner of the land, 1/5th to the financier and the rest goes to the workers.
Access to these gem producing areas is often limited especially during the monsoon seasons. When the word spreads that a miner has struck paydirt, the area is soon inhabited by treasure seekers who dig pits everywhere. These pits are usually dug very close together with somewhat thin retaining walls between them. Pits can be up to 50 feet deep and are hazardous at night to both humans and animals. Deserted pits, left with standing water, become breeding grounds for mosquitoes and the resulting malaria is constantly challenging the local governments.
Pits are normally worked by 4 workers, one to fill the baskets with "ILLAM", one to throw it to the top, one to catch it and one to take it to the washing area.
Water is a big problem especially with the high water table that some areas have. It must be constantly bailed or pumped out using petrol-operated pumps since by law, only the simplest of equipment is allowed in Sri Lanka. Only in areas slated to be submerged due to the construction of large dams for hydro-electric projects has the government allowed modern mining methods.
Although the many restrictions have hindered the progress of the gem industry in Sri Lanka, it has provided prolonged employment for thousands of needy villagers.
Once the gem gravel reaches the central washing area, it is emptied into deep conical baskets which are large enough to wash approximately 20 basketfuls of illam at a time.
The art of washing is reminiscent of panning for gold with the water carrying the mud and lighter stones through the fine mesh and over the top of the basket. This leaves the heavier materials clean and concentrated at the bottom of the cone.
The sorting is carried out by one who is expert in the recognition of rough gems. First the larger fine precious stones are removed (known as JATHI) and given to the financier of the project for safekeeping. The residual (known as TOURA-MALI) is handed to another man to ensure that nothing has been missed.
The Sri Lankan gem gravels also produce zircon, tourmaline, peridot, quartz, garnet, feldspar and a number of other stones. Diamonds, opal and emerald are not found on the island.
Thailand
Thailand is bordered by Mayanmar and Laos to the north, Laos and Cambodia and the Gulf of Siam to the east, Malaysia to the south and the Andaman Sea and Burma to the west.
It was formerly known as Siam until 1939 when it became known as Thailand.
Allied to the Japanese in the Second World War, the name was abandoned in 1945 by the pro-allied free Thai government to expediate peace negotiations with Britain but was revived in 1948.
Although Thailand has been a constitutional monarchy since 1932, political power long resided with the military regime.
In 1973, the military regime of KITTIKACHORN was overthrown by student uprising and replaced by a civilian government in 1975.
Thailand covers some 514,000 square kilometres. It's population is approximately 55.5 million. The capital is Bangkok.
Thailand is unique in that it is the regions only nation to have avoided the experience of colonial domination and has therefore been able to preserve much of its traditional society, religious traditions and ancient India-derived conception of governmental authority.
The climate is hot and humid most of the year. In the central valley, temperatures rarely go below 65 F in the coolest months (December / January) and extend upward to around 100 F from March to May.
The rainy season extends from June through September. The central valley lies in the rain shadow of the Mayanmar mountains to the west and receives annual precipitation of 40 inches compared to 220 inches in Mayanmar. The vegetation is tropical.
Gem quality Corundum are found at three major localities in Thailand and neighbouring Cambodia.
Gem Localities
Chanthaburi, Trat, Pailin
- Khao Ploi Waen : thought to be the first place in Thailand where Corundum was found.
- Bang Kha Cha Gems Found : Blue, green, yellow and black star Sapphires
- Bo Waen : Ruby only
- Bo Na Wong : Ruby only
- Wat Tok Phrom : Ruby only
- Ban Bo I-Ram : Very deep Blue Sapphire
- Nong Bon : Ruby - larger stones
- Bon Rai : Ruby - finer colours
- Pailin Fields : Ruby
Best known for Blue Sapphires ranging in colour from medium to deep blue, considered among the worlds finest.
Bo Ploi, Kanchanaburi
Considered to be Thailand's major source of Blue Sapphires.
It was discovered in 1919 and also produces the occasional yellow, pink and star Sapphires.
The Blue Sapphires strongly resemble the heat-treated blue sapphires from Sri Lanka. Most are sold in parcels of Sri Lankan goods because of the preference for Sri Lankan blue sapphires.
Phrae Province
Although this area has been known since the 1920's, it has only been worked since the 1970's. The area produces mainly Blue and Green Sapphires. The blues are a very deep inky-blue sometimes slightly violet. Large sizes 5cts or more are rare. The smaller gems are usually cut as melee due to the deep colour. Phrae Province is considered the least important in terms of production.
Mining
There are two types of mining in Thailand.
- Primitive Pit Mining
- Mechanized Mining
As a general rule, access is limited into many of the gem producing areas. At BO WAEN, BO NA WONG, WAT TOK and BAN BO I-RAM, access is very poor although a number of large mechanized mines are found. At NONG BON and BO RAI access is good and it is at these two sites, where Thailand's largest mines can be found.
In the dry season, the roads are extremely dry and dusty but during the rainy season, the roads become almost impassable except by four-wheel drive. In some cases, the mud can be several feet deep and often crudely built bridges are washed away during the summer rains.
Most of the mining in the Chanthaburi area is alluvial. If gem indications are found, the jungle can be uprooted and cleared in a matter of days. What remains is an open clay field where gems may be found at depths of only 3 to 5 feet.
When gems are found, local villages spring up quickly with housing made of regional timber and tin roofs to protect against the monsoons. Many houses are not enclosed within walls. Digging, washing and sorting are all carried out in the backyards.
Generally, one worker will remove the earth at the bottom of the pit and another will man a bamboo lift which brings the earth to the surface. After sufficient quantities have been brought to the surface, the earth is placed in either bamboo trays (similar to Sri Lanka) and washed in small man-made pools or placed in large sluice boxes, screened at one end. High pressure water is forced onto the gravel, washing away the loose clay which runs as mud and water through the screens. Washing may take up to an hour leaving pure gravel behind.
As the gravel is sorted, the sorter usually puts the gems into his mouth and when his mouth is full or the basket is empty, they are put into parcels. Sometimes they are put into plastic bags filled with water to intensify the colour and magnify the size to potential buyers.
A more costly venture in a major producing area is the use of a bulldozer. Using this method, the jungle and overburden can be stripped in a matter of hours. The remaining soft clay, can then be eroded away using powerful water hoses and the resulting mud is "vacuumed" away by the second miner and directed towards the sluice boxes.
The mud is then filtered through the screens built into the bottom of the sluice boxes leaving the gem gravel behind.
Chanthaburi is famous for its household industry in the cutting and polishing of gems. These include not only locally mined gems but also gems from around the world. Almost every home has a grinding wheel and one or more polishing wheels. Both hand and power driven machines are used.
After the preforming has been done, the final polish is placed on the stones. The stones are "dopped" and propped against a make-shift ledge with all the angles and finished polish estimated by the naked eye.
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