Collector’s Edge Minerals, Inc. recognizes and thanks the Mineralogical Record for permission to use portions or all of their original article from the “Mineralogical Record”, Volume 41, Number 1, pp. 59-67. Contributing authors were Steve Behling, Collector’s Edge Minerals, Inc.; and Wendell E. Wilson, Publisher & Editor-in-Chief of the Mineralogical Record. Collector’s Edge has made some additions and changes to the original article which are incorporated below.
Collector’s Edge Minerals, Inc. (CEMI) once again was at the forefront of a remarkable opportunity to work with a mining company to recover and prepare fabulous mineral specimens.
Emeralds have been mined in Zambia since 1928. However, hardly any specimen-quality crystals were saved until 80 years later, when the management at the Kagem mine investigated the possibility of employing a professional specimen recovery and preparation company to carry out the demanding work of freeing the beautiful emerald crystals from enclosing quartz. The resulting specimens, prepared by Collector’s Edge Minerals, Inc., made their debut in September 2009.
The Zambian emerald deposits are located in the Kitwe district in north-central Zambia, near the Kafubu River in the Ndola Rural Emerald Restricted Area, about 45 km southwest of Kitwe, in the southern part of the Zambian Copperbelt (see Korowski and Noteheart, 1978). Access from Kitwe is via a paved road for 15 km to Kalulushi, then by a poorly maintained gravel road for 30 km to the mine area. Large open-pit emerald-mining operations include the Grizzly, Chantete, Kamakanga, and Kagem concessions, together constituting one of the world’s largest producers of gem-grade emerald, ranking second in output (by value) only to the Columbian mines (Zwaan et al., 2005). Other operations in the Kafubu area include the Twampane, Akala, Pirala, Miku, Ebenezer and Mitondo mines. Nearly 20% of the world’s annual production of emeralds comes from the Kafubu area (Seifert et al., 2004).
The emerald mines in the Kafubu pegmatite field; all are located where metabasite and pegmatites come in contact (adapted from Sliwa and Nguluwe 1984; and Zwaan et al., 2005).
The source of the specimen emeralds in quartz is the open pit of the merged Kagem Fwaya-Fwaya mine and the Kagem Fwaya-Fwaya Extension F10 mine. None of the other Kagem pits have produced emerald crystals in quartz lenses.
Typical local Zambian village.
The Kagem mine is thought to be the world’s largest open-pit mine for colored gemstones. The highly mechanized open pit covers an area of about 1 square kilometer and employs about 400 men; it produces between $15 million and $30 million in emerald rough annually (Seifert et al., 2004). High levels of groundwater in the area have made underground operations uneconomical in the past because of the attendant pumping expense; hence all operations in the district thus afar have been open-pit. However, underground workings are now being developed at the Kagem mine.
The open-pit Kagem mine (the merged Kagem Fwaya-Fwaya and Kagem Fwaya-Fwaya Extension F10 pits). Note the white pegmatite veins cutting across the country rock at left.
A gentle reminder that theft is not tolerated!
Mine viewing platform.
Excavating emerald-bearing material from reaction zone.
Local Gemfields miners hunting for loose emeralds.
Despite the huge production of emerald gem rough, no crystal specimens had been produced until the operator of the mine, Gemfields PLC, entered into an arrangement with Collector’s Edge Minerals, Inc. (CEMI) to prepare and market emerald specimens for collectors and museums.
Emeralds were first discovered in the Kafubu area in 1928 by the Rhodesia Congo Border Concession Company, which later became the Miku mine (Sliwa and Nguluwe, 1984; Seifert et al., 2004). Early production was unpromising, but exploratory work was continued in the 1940s and 1950s by the Rhokana Company and the Rio Tinto Mineral Search of Africa Company. After new occurrences were located in the 1970s, gemstone exploration and production expanded radically. Extensive illegal mining in the area soon compelled the Zambian Government to declare the area a “restricted zone” and to expel the local population.
Since that time, only government-sanctioned mining has taken place there – via large-scale commercial mining and by small-scale miners on several hundred small claims. In 1980, Kagem Mining Ltd. (55% owned by the Zambian Government) was authorized to explore and mine the Kafubu concession. In 2004 the United Kingdom-listed Gemfields Resources PLC (controlled by Johannesburg’s Pallinghurst Resources) began systematic exploration near the Pirala mine, south of the Ndola River, and discovered significant emerald deposits. By 2007 Gemfields had acquired 100% ownership of two mines in the area and about 60% of the prospective strike lengths of emerald-bearing host rocks in the Ndola Rural Emerald Restricted Area. Mining began at the Kagem mine (actually a group of mines), north of the Ndola River, in 2005, and Gemfields was awarded a management contract there in November 2007. In the following June, Gemfields acquired a 75% ownership in the mine, the remainder being held by the Government.
Emerald production at the Kagem mine for the year ending June 30, 2009, was 27.6 million carats, from an average ore grade of 349 carats per ton, at an operating cost of about 40 cents per carat (Gemfields, 2009). As of 2010 the emerald rough was selling for an average of about 75 cents per carat, but high-quality rough was selling for up to $500 per carat.
Until 2008, all of the emerald-bearing ore produce at the Kagem mine was processed by large-scale crushing, washing and sorting in the adjacent processing plant.
Sorting belt where emerald rough is hand-picked from crushed schist ore (Gemfields PLC photo)
However, Gemfields executives suspected that the emerald-bearing quartz veins that are occasionally encountered in the schist might, with proper preparation, yield fine emerald specimens for collectors and museums. The opportunity to test this theory came in late 2008, when an unusually thick zone of emerald-rich quartz was encountered in the pit. This zone yielded five large chunks of quartz ranging from 2 to 35 kg, enclosing a number of gem-quality emerald crystals. Gemfields decided to withhold those boulders from normal processing and see if a specimen preparation company could be enlisted to evaluate their potential.
A block of emerald-rich quartz, before preparation
In February 2009 Gemfields contacted Bryan Lees of Collector’s Edge Minerals Inc. in Golden, Colorado, and together they developed a plan to collect, prepare and market emerald specimens from the Kagem mine. Collector’s Edge technicians visited the mine site and introduced Gemfields’ employees to advanced mineral specimen collecting tools and techniques which they could use to help maximize the intact recovery of emerald-rich quartz blocks having specimen potential. As it turned out, these tools and techniques proved useful in day–to-day gem mining work as well.
Huge tonnages of emerald-bearing rock are processed annually at the Kagem mine, but only a tiny percentage consists of the emerald-bearing quartz that is suitable for the preparation of specimens. Since March of 2009, approximately 50 kg of the highest-quality emerald-rich quartz have been shipped to Collector’s Edge Minerals for preparation. This material had been collected by Gemfields’ employees over the preceding 18 months. A larger quantity of lesser-quality, carving-grade emerald-in-quartz was also shipped. The first emerald specimens produced by the mine and prepared by Collector’s Edge Minerals were offered for sale at the 2009 Denver Gem, and Mineral Show.
Emerald bearing pegmatite bands intruding amphibolite and talc-magnetite schist.
Three major rock units of the 1.7-billion-year–old metamorphic Muva Supergroup occur in the Kagem mine area (Daly and Unrug, 1983). The uppermost layer consists of mica schist measuring from 30 to 40 meters thick. The top 10 to 20 meters of this schist unit have weathered into soil and laterite. Beneath the mica schist is a layer of amphibolite between 15 and 20 meters thick. Under the amphibolite is metabasite, a magnesium-rich and chromium-rich metamorphosed volcanic rock consisting of talc-chlorite-actinolite schist +- magnetite, which hosts the emerald mineralization (Sliwa and Nguluwe, 1984; Seifert et al., 2004). Light-colored beryllium-bearing quartz-feldspar pegmatites and epigenetic hydrothermal quartz-rich veins associated with the Pan-African Orogeny cut across all three units. Potassium/argon age-dating of muscovite associated with emerald crystals in the reaction zones suggests an age of crystallization of approximately 447-452 million years ago (Seifert et al., 2004).
Emeralds occur at the Kagem mine mostly in metasomatically altered phlogopite-rich zones up to 3 meters wide that rim the numerous pegmatite dikes and quartz-tourmaline veins (genetically related to a granite pluton at depth) cutting through the talc-magnetite schist country rock. They may thus be classified as “schist-type” emeralds, like those from Brazil, Madagascar, Russia and South Africa (as opposed to “non-schist” emeralds from Columbia and Nigeria). The best emeralds occur where the reaction zone is near intersection of the pegmatite veins and quartz-tourmaline veins (Zwaan et al., 2005). The majority of the emerald mineralization took place at temperatures of 360 deg. to 390 deg. C and pressures of 400 to 450 MPa.
The Kagem emerald concession covers an area of approximately 46 square kilometers within a 200-square-kilometer productive zone. Within this area are six known belts of talc-magnetite schist totaling over 13 km in strike length. One of these belts, the Fwaya-Fwaya-Pirala belts, is currently the main focus of emerald production at the Kagem mine. Core drilling along this belt has established a strike length of 1,720 meters. As of 2005, emeralds had been mined to a depth of about 60 meters, but field surveys, chemical analyses and structural studies indicate that the productive zone extends significantly deeper (Zwan et al., 2005).
The emerald appears to have formed as a single generation of growth within the schist (and not in the pegmatites.) The schist and associated Mg-rich metabasic rocks provided the necessary elements for the crystallization of emeralds, including 3000 to 4000 ppm of chromium and a trace of vanadium, these two elements being the source of the emerald-green color. The chromium is concentrated primarily in chlorite (up to 1.5 wt.%) and magnetite (up to 13.6%) (Seifert et al., 2004).
The majority of the emeralds mined at the Kagem mine occur within the friable schist reaction zones where they are easily removed by crushing and processing of the rock. However, emeralds suitable for preservation as mineral specimens are found predominantly within quartz boudins or lenses within the schist and on or near the contact with the pegmatite. Thus they are unusual (perhaps unique) in being “schist-type” emeralds that are hosted in quartz rather than schist.
At a late stage in the pegmatite emplacement, after free-growing emerald crystal had formed in open pockets in the schist, silica-rich fluids were introduced into the reaction zone. These fluids precipitated quartz in lenses up to 30 cm thick, completely enclosing the free-growing emerald crystals and filling the open pockets. This encapsulation of the emerald crystals in quartz protected them to some extent from further stress and chemical activity. Nevertheless, minor tectonic movements fractured some of the embedded emerald crystals in the thinner quartz lenses, and the fractures were then filled by quartz. The fractures commonly follow a poor (0001) cleavage, but some crystals show a second set of parallel fractures at an angle to the c axis. Emerald crystals were best preserved in the larger, more massive quartz lenses (up to 30 cm thick) that were better able to resist plastic deformation under tectonic stress. Unfortunately, quartz lenses thicker than about10 cm are very scarce, so the opportunities for recovering high-quality emerald crystal specimens are rare.
View of quartz stringers in pegmatite.
In-situ quartz lens.
In many of the quartz lenses the emerald crystals are concentrated on one side of the pod, suggesting that gravity may have caused them to accumulate at the bottom of the cavity. Also, movement of the siliceous fluid may have caused the preferential orientation of some of the emerald crystals, so that they tend to lie flat parallel to the boundary of the pod, and are also aligned along the direction of the flow.
The emerald variety of beryl (Be3Al2Si6O) is generally restricted by gemologists to those specimens deriving their color from trace amounts of chromium (and possibly vanadium). Kagem mine emeralds contain 0.26 to 0.86 weight percent Cr2O3 (Zwaan et al., 2005), the color intensity of the darker crystals correlating well with Cr content; green beryl with as little as 0.04 weight percent Cr2O3 were also identified. Some crystals contain up to 0.23 weight percent Cr2O3. Vanadium content is uniformly low (around 0.02 wt. %), and thus it is unlikely that vanadium contributes much to the color of the crystals. The best crystals exhibit an intense, classic emerald-green color with a faint bluish tinge. Colorless, pale green, blue –green and blue beryl crystals have also been found. Some crystals are color-zoned, with a paler yellowish green to greenish blue core surrounded by a deep green rim (Zwaan et al., 2005), but other crystals may be deep green all the way through.
The best crystals have a mirror-smooth luster and a high degree of transparency. In other cases, inclusions of black phlogopite render the crystals darker and less transparent. Matrix specimens may show emerald crystals resting on white quartz or on black phlogopite schist.
The sharp-edged, well-formed crystals consist of a simple hexagonal prism terminated by the (001) basal pinacoid. The habit tends to be elongated; crystals up to 14 cm in length and 2.5 cm in width have been recovered. However, crystals of common beryl up to 30 cm long have been reported from within the pegmatite in the Kagem concession (Seifert et al., 2004), and a local mine manager reported that during the 1980s large emerald crystals up to 60 cm long were found at the Kamakanga Old Pit (Zwaan et al, 2005).
The emerald crystals are found predominantly near the boundaries between the quartz lenses and the schist, often lying parallel to the boundary, with a little quartz between the crystal and the contact with the schist.
A remarkable number of other species have also been identified as inclusions in Zambian emeralds, including actinolite, tremolite, chlorite, dravite, apatite-(CaF), magnetite, hematite, quartz, fluorite, magnesite, siderite, dolomite, calcite, ankerite, niobium-rich rutile, pyrite, talc, zircon, barite, albite, lepidocrocite, glauconite, quartz, chrysoberyl, margarite, muscovite, biotite, brookite, tourmaline and crysotile (Huong, 2008; Zwaan et al., 2005; Milisenda et al., 1999; Moroz and Eliezri, 1999; Graziani et al., 1983; Koivula, 1982, 1984; Gubelin and Koivula, 1986). One-phase, two-phase and three-phase fluid inclusions are also present (Zacharias et al., 2005).
Crystals found scattered within the largest quartz lenses (up to 30 cm thick) tend to be the finest and best preserved. The emerald crystals that extend into the quartz lens with their bases at the schist contact are typically darkened by phlogopite inclusions near the base and become gemmier as they extend into the quartz.
Emerald crystals of a generally lower quality occur in the more common thin (3 to 8 cm) quartz lenses, which can run 20 to 30 cm in length. The crystals and the surrounding quartz both commonly suffer from extensive fracturing. The quartz has a more granular quality and is more opaque because of the fracturing. Kagem emeralds are also more opaque, both from phlogopite inclusions and from fractures. It is common to see many fine parallel white lines of incipient or re-healed fractures running through the emerald crystals. The luster of these crystals ranges from resinous to bright on the better examples, and waxy to dull on the poorer ones. Yellowish to brownish iron staining is present within fractures throughout the lenses. Elongated gemmy emeralds do exist within these thinner pods, but have usually been fractured into pieces by tectonic forces and cemented with quartz. The fractures vary in thickness from hairline width to several millimeters. Some of these fracture crystals appear bent or curved.
A chunk of typical quartz containing emerald crystals
Emerald crystal, 10.5 cm, in quartz; total specimen size 13 cm x 10.5 cm x 6 cm. MIM Museum collection, Beirut, Lebanon. Richard Jackson photo.
Emerald crystals occurring within the phlogopite schist (below) ranges in quality from gem-clear to almost completely filled with phlogopite inclusions. The color of the beryl is still very good, but it is generally darkened by inclusions of black phlogopite, or appears dark because of the dark schist showing through from behind the more transparent crystals. The crystals show a high luster on the well-formed faces, and waxier luster on undulating faces.
The schist-hosted emeralds occur as single crystal specimens but more commonly as flat-lying clusters showing some preferred orientation, and as radiating sprays. The schist-hosted specimens commonly contain many crystals. Of the specimens received by Collector’s Edge there was only one fully transparent schist-hosted emerald. It is unusual that such a specimen was saved, because most of the emeralds in the friable schist have been processed for gem rough. This particular specimen was found in a much harder, sturdier schist matrix and hence was saved as a specimen.
A plate of schist-hosted emeralds – though gem quality in some areas, a few are suitable by themselves as individual specimens.
The preparation process for the emerald specimens is slow and tedious because of the brittle nature of the emeralds and the hardness of the enclosing quartz. A single quartz-enclosed specimen can take several months to prepare. The preparation work is entirely mechanical; no chemicals are used, as any chemicals that could remove the quartz would surely damage the beryl as well.
Collector’s Edge laboratory manager Robert Lorda displaying a block of quartz containing emeralds
The first step is a visual assessment of the “raw” quartz/emerald block. Laboratory technicians at Collector’s Edge take note of all the emerald crystals exposed at the surface of the sample. Using exposed emerald crystal as a starting point, an intense light source is focused into the translucent quartz block around the area of the crystal. Emerald crystals embedded in the quartz will glow faintly, allowing the technicians to estimate their orientation and approximate length. The presence of emerald crystals not exposed at the surface can also be detected by this “green glow” technique.
The lab technicians then carefully plan the initial diamond saw cuts and mark them on the surface of the quartz boulder with grease pencils. The larger quartz boulders and lenses are then cut up into more manageable sizes, and the trimmed quartz blocks are once again subjected to visual examination with the intense light source. By this method one is only able to “see” 3 to 5 centimeters into the quartz blocks, so when exploring for emerald crystals within the larger quartz pieces it is necessary to trim and re-examine them many times to make sure that none of the terminated emerald crystals are accidentally cut by the diamond saw.
Additionally, during this exploration phase, a decision is made regarding the best and most aesthetic final orientation of the potential emerald specimens. It is important to keep this vision of the final specimen in mind, so as not to saw away any quartz which might later be wanted as matrix for the finished piece. Every effort is made to preserve more than one terminated emerald crystal on a given piece of matrix. Sadly, very few specimens have been encountered which have a geometry that will allow more than one crystal to be kept on the same piece of quartz matrix.
Emerald-bearing quartz after initial removal of material. Specimen is unusual in that it shows multiple emerald crystals that potentially can be mechanically etched out of the quartz to result in a multi-crystal emerald specimen.
Quartz containing emerald crystals after more preparation.
Once all the potential specimens have been identified, oriented and trimmed into their individual quartz blocks, the painstaking work of removing the enclosing quartz is begun. The Collector’s Edge laboratory technicians utilize various micro-pneumatic tools to slowly peel away the quartz, 5 to 10 mm at a time. The impact made by these small power tools can produce unwanted fractures in the quartz matrix, so whenever necessary the quartz is vacuum-impregnated with an optical-grade epoxy to stabilize it.
Quartz boulder revealing multiple emerald crystals as preparation continues.
As the emerald crystals are gradually exposed from the quartz a temporary protective coating of epoxy resin is applied to their surface. This coating shields the crystals from the impacts of any abraded quartz particles. Because of the brittleness of the emeralds and the tenacity of the massive quartz, extreme care must be taken during the final stages of quartz removal. The micro-pneumatic tool chosen for the final stage of specimen preparation is more precise, and the removal of the final layer of quartz from the emerald is done under the microscope. The technicians also employ micro-air–abrasive techniques to remove quartz in the final stages.
Quartz containing emerald crystals as preparation continues to reveal several fine crystals.
Once the quartz matrix has reached its final configuration, the laboratory technician uses air-abrasive techniques to remove all of the visible epoxy resin from the surfaces of the quartz and the emeralds. The specimens are then finished and ready for sale. By agreement with Gemfields PLC, Collector’s Edge Minerals, Inc. is the sole world-wide distributor for the Kagem mine emerald specimens.
Preparation completed, revealing multiple, exquisite emerald crystals! Specimen is approximately 14 cm x 12 cm x 7 cm and resides in the British Museum.
Quartz containing emerald crystals as preparation progresses. Red marks indicate trimming areas of excess quartz.
Temporary epoxy coating to protect emeralds as preparation continues.
Further preparation and addition of temporary epoxy protective coating.
Final emerald specimen after preparation. Approximate size 11 cm x 11 cm x 6 cm.
Quartz containing single emerald crystal in initial preparation stage.
Emerald specimen coated with temporary protective epoxy coating and matrix marked
(red line) for removal.
Final emerald specimen after preparation. Final size approximately 4 cm x 4 cm x 4 cm.
Gem quality emerald, approximately 5.6 cm x 3.3 cm x 3.2 cm.
Emerald crystal, 10.5 cm, in quartz; total specimen size 13 cm x 10.5 cm x 6 cm. MIM Museum collection, Beirut, Lebanon. Emerald is of exceptional length, mostly gem quality with perfect termination. Richard Jackson photo.
Emerald specimen, size 8.7 cm x 5.3 cm x 3 cm. Collector’s Edge specimen.
Excellent multiple emerald crystal specimen, size 6.5 cm x 6 cm x 3.5 cm.
Multiple emerald crystal specimen showing combination of “clean” emerald and end of one crystal heavily impregnated with phlogopite inclusions. Size 7.7 cm x 6.4 cm x 3.0 cm.
Emerald specimen showing crystal in both clean quartz and phlogopite schist wall rock. Size 4 cm x 7 cm x 3.5 cm.
Multiple emerald specimen, size 9 cm x 7.5 cm x 8 cm.
Emerald crystals encased only in quartz matrix; size 8 cm x 8.5 cm x 5.2 cm.
Excellent specimen showing emerald crystal fractured by tectonic movement with fine quartz rehealing of breakage; size approximately 18 cm x 11 cm x 8 cm.
Schorl is common within the quartz lenses, occurring as thin, elongate black crystals with high luster. The schorl occurs primarily as aligned crystals within the schist and as thick bands at the contact between the schist and the thinner quartz lenses. It is commonly seen intergrown with schist-hosted emeralds. No collector-quality schorl specimens, or combinations of emerald and schorl specimens, have been discovered. The emerald/schorl specimens will most likely be relegated to carving rough.
Other minerals found in the Kafubu emerald area include allanite (small euhedral crystals in rock), amethystine quartz (in a single 10-cm-wide vein), bertrandite (7-mm crystal aggregates replacing beryl), columbite-(Mn) (prismatic euhedral microcrystals), monazite (euhedral microcrystals), epidote, gahnite, phenakite (crystals to 1 cm enclosed in beryl), plumbomicrolite (brownish yellow crystals to 1 mm), and xenotime (euhedral microcrystal inclusions) (Seifert et al., 2004).
The beautiful green Kagem mine EMERALDS are cut by skilled gem cutters into faceted stones with incredible color and brilliance. These gemstones look stunning in rings, pendants and earrings! The emerald can also be fashioned into cabochons and a variety of other polished forms suitable for use in jewelry and perfect for rare gemstone collections.
Hand sorting emerald rough.
Facet quality crystal emerald rough separated during sorting.
Emerald rough being faceted.
Pear-shaped 1.68 carat, 10mm faceted emerald
Group of faceted emeralds
A unique, one-of-a-kind table produced using an emerald bearing quartz slab (almost a meter in length)
Currently no further emerald bearing quartz lenses have been located at the Kagem mine.
Collectors Edge Minerals Inc. recognizes and thanks the Mineralogical Record for permission to use portions or all of their original article from the “Mineralogical Record”, Volume 41, Number 1, pp. 59-67. Contributing authors were Steve Behling, Collector’s Edge Minerals, Inc.; and Wendell E. Wilson, Publisher & Editor-in-Chief of The Mineralogical Record.
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