Article Featured in Jewellery News Asia (January Issue)

Deep mining for gems in Sri Lanka: A myth or a reality

 Professor P.G.R.Dharmaratne

(PhD,FGG,FGA)

Gem mining in Sri Lanka goes back to many centuries.  In the early days, there was no systemic mining. The systematic and legal mining based on licenses issued by the State came into effect around 1970 with the formation of the State Gem Corporation. Gem mining and exports increased by leaps and bound. For a few decades, deep mining for gems have been discussed.

Traditional gem mining (Artisanal mining)

Gem mining over many centuries and particularly the rapid increase in activities since the formation of the State Gem Corporation saw the depletion of easily accessible gem deposits at a higher rate.  Gems have been mined in alluvial or secondary gem deposits.  About 20% of the island is covered with such deposits. This is probably the highest percentage of alluvial gem deposits for any country. The depth of these deposits varies from surface to sometime 30 – 40 meters.  During the early stages, on the surface or near surface deposits were mined by digging pits with sizes varying up to 2 x 4 m in section.  As the near surface deposits depleted, gradually deep deposits were begun to be mined and it increased the cost of mining due to the increased cost of timber, water pumping etc.  The nature of our mining is such that digging of pits always leaves room for many generations to dig in the same land over and over again.  What is left over by previous miners may be found to be fruitful for the subsequent licensees So when one licensee is unsuccessful and gives up another will turn up and start mining again and  find many valuable gems.  If, however the gem gravel bed is scooped up totally by mechanized methods such as what Australians and Thais are doing, nothing will be left for another gang to work on. That is why although many hundred thousand people  were engaged in mining gems  for over many decades there is still a steady flow of gems from this tiny island.  Thus not allowing mechanized mining has proved worthwhile for the continuity of the flow of gems, as well as employment in the gem mining industry. 

Mining of primary sources(In-situ deposits)

Alluvial or secondary gem deposits form when primary gem sources are subjected to degradation due to natural causes such as rain and wind etc over many centuries.  The broken down rocks consisting of gemstones  were transported over many miles over millions of years and these have formed alluvial beds which have been covered  with subsequent depositions of clay, sand, etc.  The waterways take different courses  over the years leaving behind alluvial formations and at the bottom of this  is the gem gravel layer, lying on the decayed base rock known as “Malawa” or  Unweathered base rock  called “Parugala”.  The very fact that the alluvial gem deposits are widely distributed over the island from Himbutana in the west to Galoya in the east and Horowpathana in the north- central to Matara in the south,,show that there must have been numerous primary sources. In fact nine tenths of the country’s rock types are metamorphosed Precambrian rocks of which the central Highland Complex is identified as “high potential” gem bearing rocks.

It is not prudent to think that all such primary sources have decayed and transported to form alluvial deposits.  There may be many source rocks near surface and subjected to different degrees of weathering and erosion.  Some may be yet unexposed and un-affected by natural elements.  The existence of weathered source rocks  were proved when accidental findings resulted in gem rushes at Hatarabage off Balangoda (1985-sapphire) Haldumulla (1987-sapphhire), Akaralla (2004- Aquamarine) and kahawatta(2005-yellow topaz)  etc. Such primary sources were haphazardly mined out in the same way as digging pits in alluvial deposits.  The experience with previous accidental findings show that at each location gems in the form of perfect crystals were found within a confine of 1 to 2 square meters and the quantity found at such places were so great that people believed them to be buried treasures of ancient kings.  In the rush to just pick up gems from such findings, there were several casualties.  Having heard of the great wealth generated, pits were dug up in the vicinity by many invaders, but it  yielded no more similar occurrences because they did not follow any scientific basis for explorations.

Deep mining

Research programme carried out at the Department of Earth Resources Engineers of University of Moratuwa, during 1990s have proved the existence of source rocks in the areas of  Koslanda, Haldummulla, Badureliya and Buttlla etc(Figure 1). Mining of such deposits need to be carried out in a different way as compared to the artisanal mining adopted in alluvial deposits in Sri Lanka at present.  Unlike in the case of alluvial deposits, gem mineralized zones in the area concerned are not flat bedded in nature but either dykes (vertical pipes) or vertical or inclined veins.  To expose such a gem mineralized zone, a vast area will have to be excavated and once such a zone is found, mining will have to be continued laterally and/or vertically depending on the nature of the formation. 

It must be noted here that in many countries where gemstones are found, not only alluvial gem deposits but also source rocks are mined.  Diamond in many African countries, Russia, Canada etc, Tanzanite in Tanzania, Tourmaline, Emerald, Aquamarine and garnet in Brazil etc are some of the examples. Diamonds are mined at depth of 2-3 kilometers while some tanzanite mines are over 3-400m deep.

Therefore, the extent of untouched in-situ sources of gem in Sri Lanka could be enormous.  Depleted alluvial deposits due to repeated mining generate less and less quantity of gems but if action could be taken to initiate mining of primary sources, the supply of gems could be increased tremendously.  What is required today is to immediately start working in some of the places where the research work has been completed to prove the validity of research finding.  Once proved economical at the initial sites, detail work at similar localities can be done and mining extended. 

So one must remember what is referred to as deep mining is not the mining in alluvial deposits, but in the source rocks where gems have formed and are still intact. Figure 2 shows a proposal made to mine a deep deposit at Kollona in 2008.

COLOUR ENHANCEMENT OF TOPAZ WITH SPECIAL ATTENTION ON SURFACE DIFFUSION

Topaz is a common gemstone and that has been used for centuries in Jewellery. Chemical formula of topaz is Al₂SiO₄(F,OH)₂. The maximum F^- content is around 21%. The variations in amount of OH^- and F^- could cause slight variations in the physical properties like the refractive indices and density, in stones mined from different localities.

A chain like structure of connected irregular octahedrons controls the structure of topaz. These octahedrons have aluminum in the middle surrounded by four oxygens. Above and below the aluminum are the hydroxide or fluoride ions. The chains of octahedrons are held together by isolated silicate tetrahedrons but it is the octahedron chains that give topaz it’s crystallizing shape.

Colour inducing methods of topaz

Topaz most often occurs colourless, but can occur naturally in a variety of colours such as pale blue, yellow, brown, orange, and pink etc. Colourless topaz is cut and polished and subjected to diverse physical and chemical treatments to induce colour.

1. Irradiation and heating method

Widely used colour inducing method for topaz is irradiation, followed by heating to produce their blue colour.  The process of irradiation involves exposure of a specimen to one of a variety of radiations. For irradiation of topaz, different treatment methods are being used. These methods consist of, gamma-ray irradiation treatment,linear accelerators treatment, nuclear reactor treatment, and combined treatments.

In this method the colour enhanced gemstones are produced by utilizing a thin-film of optical coating. This process is unique, utilizing various minerals, oxygen and high vacuum. The coating is deposited on the pavilion. No radiation or harsh chemicals are used that would harm the environment. The thin film coating is durable and hard. Different colours are produced by this method such as, blue, green, pink, red, pinkish orange and many more colours.

3. Surface diffusion method

      Surface diffusion is a recently used process for inducing colour on topaz. In this method, a slurry of the powder of diffusion material is painted on the surface of the stone, and then left to dry. Then these stones are subjected to heat treatment. One or more transition elements, present in the diffusion material, will be diffused into the stone and induce a colour. This is a low cost method, and has some advantages.

Surface diffused topaz

Surface diffused topaz made its first appearance in early 1998. Surface diffusion process for topaz, requiring a temperature of 850 ⁰C to 950 ⁰C, can be applied only to a very durable stone. Surface diffused topaz has a colour that is confined to a microscopically thin layer at and just below the surface, with colours described as blue to greenish blue or emerald green or different colour shades.

When comparing with irradiated topaz, surface diffused topaz has some advantages, such as

a)                  Unlike most irradiated topaz, diffused topaz has no residual radioactivity, thus requiring no costly cooling down period while stones decay to safe levels.

b)                  This process allows for colours other than blue.

c)                  Colours of diffused topaz will not be lost at higher temperatures while the colours of natural or irradiated topaz will fade away at a temperature of several hundreds degrees Celsius.

d)                 The cost is low.

Only disadvantage of colour-diffused topaz is that it loses colour if a stone is chipped or re-cut.

Properties of surface diffused topaz

The physical properties (specific gravity, optic characteristics etc.) and visual appearance of surface-diffused topaz were typical for topaz. However, the colour was unusual for a topaz, as an example bright green topaz doesn't exist in nature, while the refractometer reading indicated the RI above 1.80. Microscopic examination shows the mottled surface of bluish green colouration and the white facet junctions of surfaces on the backside when viewed with diffused light.