The Technical File “Basic concept for valuation of common gemstones in Malawi” featured below was initially published in Malawi’s Mining Review Issue Number 27, July 2015.
The Technical File series is written by a leading Mining and Environmental Management Expert, Grain Malunga (FIMMM).
Basic concept for valuation of common gemstones in Malawi
Malawi is believed to be losing foreign exchange earnings because of lack of support to artisanal-small scale mining. Usually gemstone miners are equipped with poor knowledge of gemstone mining, grading and pricing. This paper tries to put into perspective basic concepts of identification, grading and pricing of gemstones.
Gemstones are inorganic minerals that are rare, beautiful and durable. They become precious when cut and polished. They are desirable if they have good colour, good shape, good size and good clarity.
In Malawi common gemstones are sapphire, ruby, aquamarine, tourmaline, zircon, amethyst and agate.
Properties of gemstones
Gemstones can be distinguished from synthetic materials through an analysis of their properties. These may be chemical or physical properties. Synthetic materials include glass and plastic.
Gemstones are a composition of chemicals expressed by elements and relative number of their atoms. The following are examples of chemical formula for gemstones:Chemical resistance
Durability of gemstones includes resistance to chemical attack. It is also important to note that a few gemstones may be prone to chemical attack. These include pearls and apatite.
The use for chemicals in gemmology is limited because it is destructive especially for carbonate materials such as pearls which effervesce. Use of hydrochloric or nitric acid help detect plastic limitation. Use of micro chemical tests under a microscope or magnifier is favoured as small acid spots are used.
The use of crystals helps to identify certain gemstones, especially those that formed in caves, voids or fractures. Crystals are grouped into classes according to symmetry and number of corners (shapes). 32 shapes or forms have so far been identified.
Here are some of the terms used to describe crystals:
Prismatic: a crystal with an elongated prism form, e.g. beryl.
Acicular: needle-like, e.g. tourmaline.
Columnar: a number of prismatic crystals in parallel growth, e.g. quartz.
Radial: crystals radiating from a common centre, e.g. tourmaline.
Concentric: crystals form layers around a nucleus, e.g. malachite.
Massive: no directional features (such as crystal faces) are exhibited, e.g. turquoise.
Colour and streak
Gemstones can be identified by their colour due to presence of impurities or structural peculiarity. Others produce coloured or colourless powder when scratched or drawn across a plate porcelain plate. This type of powder colour is called streak. Pyrite produces black colour while malachite produces green colour.
Many well-established gemstones are simply colour varieties of a mineral. A few examples of many are given here:
3BeO Al203 6SiO2
|Tourmaline B, Al, etc., silicate||Red
Some gemstones have the ability to absorb light differently when it passes through the gemstone in different directions during rotation. Those that exhibit two colours are said to be dichroic while those that exhibit more colours are said to be pleochroic. Tourmaline exhibits dichroism.
The appearance of a gem as it reflects light is called lustre. Lustre can range from glassy (vitreous) to metallic. Aquamarine is vitreous while gold is metallic. Common examples are shown below:
|Vitreous (like glass)||the lustre of ruby, emerald,|
|Resinous (like resin – greasy)||the lustre of amber.|
Quality of gemstones
Quality of gemstones is determined by colour, size, clarity and shape.
Colour contributes about 60% of the value in any coloured stone. Examination of colour is done in various kinds of light including under fluorescent, incandescent and sunlight. Good stones show good colour if lit above white paper. It has been proved that stones that are back lit and show good colour often display dark colour after being cut so be careful about stones displayed with back light.
Remember use the “white-paper test” for colour quality and density because dichroism and pleochroism may affect the desirability of the colour you prefer in a gemstone.
The size of a gemstone determines the size of cut gemstone that will be a final product. A common rule is that a 1 gram gemstone will produce a 1 carat cut stone. 80% is lost. Please note that 1 gram is equivalent to 5 carats. The bigger the stone the more carats it will weigh and more value will be obtained from it. Coloured stones are measured in millimetres and sold by carats.
The rule is “never attempt to cob a stone with wrong tools since you will break it into useless pieces and you will introduce more flaws”. You are advised to use a cobbing hammer.
Clarity in gemstones is affected by intensity of flaws in them. Typical flaws include cracks, voids open cleavages and inclusions. These compromise appearance and structural integrity of gemstones. Flaws affect light penetration into gemstones and absorb light and interfere with the optics of a gem. Flaws like inclusions of water bubbles and air voids help to identify origin of gemstones and differentiate them from synthetic material. The less the flaws, the higher is the price of gemstones and they can be removed during cutting.
Good stones for cutting should be blocky, chunky and rounded. Any stone outside this description will lead to a lot of waste during cutting or faceting. Flat or twisted faces can as well be sold as industrial material for recovery of elements such as beryllium in beryl or zirconium in zircon.
Simple Toolkit for rough gemstone identification and valuation
Basic tools required for rough gemstone identification and valuation can be obtained under USD4,300.00 or MK2,000, 000.00. These are; Cobbing hammer, Loupe, Lamp with solid opaque shade, Fluorescent Light, Incandescent Light, Tweezers, Glass of water, White paper, Millimetre gauges, Optivisor binoculars, Gram scales model PJ-600, Carat scales, Chelsea filter, Pocket diffraction grating scale, Dichroscope, Gravity Travel Kit, Gem Shovel, Analogue Faceting machine, Ultratec facet saw kit
Grading and pricing
Gemstones are graded according to quality and size. These determine the pricing of gemstones. Remember the price of gemstones takes into account loss during faceting, colour intensity and uniformity and clarity. Generally tradable rough gemstones should be more than 1 gram (5 carats) and with less inclusions. The price of gemstones is not fixed and depends on willing seller and willing buyer. Reverse pricing is usually a good method. The Text Box below tries to illustrate the rough price of a ruby:
Adding value to ruby
The process above indicates that a good value of gemstones is in the cut stone. Therefore there is need for miners to go into value addition. Taking into account quality factors given above, a tentative grading system can be devised by miners or traders as per the following example:
|GRADE||COLOUR||CLARITY (%)||SIZE (in gm)|
|I||Deep colour||>90% clear: Minor visible to no visible inclusions||> 1|
|II||Some colours with one dominant deep colour||75 to 80% clear: Light inclusions that may not be avoidable in cutting||1-3|
|III||May have one or more colours||60 to 75% inclusions and will be clouded when cut||>3|
|IV||May have one or more colours||Heavily included with rock like texture. Good for cabochon||>3|
Prices for common rough gemstones of Grade I type have an average cost given in the table below. These are indicative prices and selling will be on willing seller/willing buyer arrangement.
|GRADE I ROUGH STONE||PRICE/GRAM (US$)|
Gemstone business requires a lot of experience and knowledge in understanding the quality and value of stones. The miner gets a smaller share of the whole cake and therefore there is need to form up cooperatives in order to benefit from economies of scale and be able to acquire lapidary equipment for value addition.
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found this very helpful thank you