By T Piyadasa-Former director general of the National Gem and Jewellery Authority
Article reproduced with kind permission from the National Gem and Jewellery Authority
Botswana is 581730 Km sq in area,situated in the South African Plateau.It is surrounded by other countries- the republic of South Africa to the south-east and south. Namibia to the west, Capravi to the north and Zimbabwe to the north-east. Igneous activity involving the intrusion and extrusion of very different types of rock occurred throughout the countries geological history. It has large deposits of iron, copper, gold, diamonds and other minerals.
The following are the descriptions of the stones they issued on stamps:
Chemical composition- silicon dioxide
Hardness- 7 on the Mohs scale
Agate forms when gas bubbles trapped in solidifying larva become filled with alkali and silica bearing water .The alkali attacks the iron in the surrounding lava and bands of the resulting iron hydroxide are created in the gel , which loses water and crystallizes, leaving the bands intact. Many Agates cut in cross section ,reveal striking forms.
The grey and white banded type is the most common variety of ordinary agate. Due to its hardness large pieces of flawless agate are used for making mortar and pestles for the chemical industry. However because of their attractive appearance , the larger Botswana specimens are used for manufacturing ornamental ashtrays.
2) Moss Agate:
A variety of agate characterized by small dendrites of iron oxide or an iron rich mineral .Egg shaped samples in which the agate is semitransparent are quite popular as a household ornament.
3) Agate -Botswana Pink Agate:
Agate is the concentrically colour banded variety of chalcedony . The fine grained or crypto-crystalline equivalent of quartz. Unique to Botswana are the specimens with a pink banding .They form within oval shaped cavities (Geodes) in rocks and the banding usually reflects the cavity shape .Agates occur in Eastern Botswana at Bobonong where they are crystallized within geodes in a volcanic rock known as Basalt.
4)Copper- Nickel Ore:
Crystal system- cubic
Chemical Composition -Cu
Hardness 2.5 to 3.0
Copper takes a number of interesting and attractive forms ranging from cubes , octahedral and dodecahedra. Crystals are often twinned and the metal is highly ductile and malleable. The luster is metallic and the streak is pale red .Copper is a good conductor of heat and electricity .Occurring not commonly in basic erotrusive rocks. This mineral group includes two of the more common copper and nickel sulphide which are chalcopyrite and pyrrhotite.
The stamp shows a sample of the ore taken from the Copper Nickel mine at Northern Botswana.
Crystal System -Trigonal
Chemical composition -Silicon Dioxide.
Silica is the most abundant oxide in the earth’s crust ,and its crystalline form Quartz, probably exhibits the greatest variation in colour and shape of all minerals.
Variety and Trade Names:
Rock Crystal- Transparent colourless
Smiky Quartz- Transparent
Rose Quartz- Semi transparent to translucent
Aventurine Quartz- Transparent to opaque
Rutilated Quartz- Quartz with rutile needles
Milky Quartz- White to very light grey colour
Star Quartz- Asteriated Quartz.
This “Stalagmitic ” arrangement of clear , small, pure quartz crystals illustrates growth in a geode. The sample was collected at Bobonong where this type of quartz is known as “finger quartz” .This mineral has many industrial uses eg building, glass making and as a flux in smelting.
The yellow or honey coloured variety of quartz is known as citrine , the colour may grade into smoky brown .The colour is due to the presence of iron. If the crystals are transparent , citrine is used as a gemstone and is sometimes called false topaz. It is rare in Botswana and is found only around Bobonong.
The purple variety of quartz is known as amethyst .Its colour is generally attributed to the presence of small amounts of manganese within the crystals. Nicely shaped specimens have generally formed in cavities and fissures in a rock. Amethyst is found in several parts of eastern Botswana.
8) Moss Quartz:
It has a dendritic pattern do the presence of thin, green ,chlorite seams.. Here the quartz is composed of numerous ,small, milky white crystals . It is occasionally used in semi-precious jewellery, particularly if the quartz is clear.
Hardness 3.5 to 4.0
Chemical composition- NaCaAlSiOH2O
Crystal System- Monoclinic and Triclinic system
Stilbite is a variety of the family of minerals collectively called the “Zeolites” .They generally grow in the cavities in Volcanic Rocks, such as Basalts of Eastern Botswana. Stilbite forms aggregates of shelf like crystals typically represented on the stamp. It has no economic value.
Marble is composed of recrystallized calcite or dolomite with a coarse to medium grain.
Marbles have always been valued for their attractive appearance and are used extensively as a facing stone in the building industry. The rocks are generally delicate green or yellows with white bands, although pink and white varieties are also seen. Colour banded marbles are found in Moshanneng in Southern Botswana.
This variety of asbestos is a fairly common mineral throughout Botswana and occurred in sufficient quantities at Moshaneng to be mined. Asbestos is always fibrous with long hair-like individual fibres. It is generally white , but green , brown and blue varieties are known. It is used in making insulating and fireproof materials.
Crystal Structure- Cubic
Chemical composition- Carbon
Diamond is the hardest mineral on earth. Its exceptional luster and brilliant fire has made it the most highly prized of all gemstones.
Diamonds are found in the rocks known as Kimberlites which forms pipes like bodies in the Earth’s crust. Botswana has opened its first diamond mine at ORAPA .Twining is common but cube forms are rare. The colourful diamond may be white, yellow,green,blue,pink,red,brown or black.
Chemical Composition- Silicon Dioxide
Crystal Structure- Trigonal
Jasper is a massive fine grained variety of chalcedony. It occurs in shades of brown, grayish blue, red ,yellow and green and mixtures of these. The area around Bobonong has jasper bearing gravels.
This blood red mineral is another variety of cryso, crystalline silica, brown, which are occasionally uniformly banded. It is a favorite semi-precious stone for use in local jewellery.
Hardness- 3.5 to 4.0
Chemical Composition- CuFeS
Crystal System- Tetragonal
Chalcopyrite is an important ore of copper and the most copper bearing mineral. Chalcopyrite with a metallic and sometimes iridescent appearance from its tarnish , is usually brassy yellow in colour with a greenish -black streak. In Botswana it forms part of the massive sulphide at the Pikwe mine. It has a metallic lustre