Rare Gemstones

Chrysoberyl is a beryllium aluminate mineral occurring in pegmatites. This gem possesses a substantial hardness of 8.5 on the Mohs scale which is inferior only to that of Diamond as well as both Ruby and Sapphire. Traditionally associated with wealth and good luck, Chrysoberyl is decidedly rare and remains somewhat a collector’s gem.

The chrysoberyl variety Alexandrite, one of the most elusive of all precious stones, was first discovered in in gravels of the Sanarka river in the Ural in 1830. As the gem’s discovery coincided with the twelfth birthday of the then crown prince Aleksandr Nikolayevich, the later Tsar Alexander II of the Romanov dynasty of Russia, it was named in his honour. This stone fittingly combines the imperial colours of Russia Red and Green via the ‘Alexandrite’ effect and was consequently popularized by the Imperial Jeweler Fabergé. Both, the particular colours and the effect are due to the trace element chromium which replaces minute amounts of aluminium inside the crystal lattice. The ideal colour shift runs from a ruby red to emerald green but is so rare it is practically never seen. In reality a rather a bluish green in daylight or fluorescent lighting to purplish red under incandescent light is seen as the best colour change. According to colour the finest Alexandrites come from the Ural Mountains along the banks of the river Tokovaya, 90km northeast of Yekaterinburg, where they occur in Phlogopite schists alongside Emerald. Unfortunately, these stones are only very rarely clean. The most productive source of gem-quality material has in recent times been Hematita in Minas Gerais, Brazil. This source has delivered faceted stones of up to 20ct. Their colour shifts from an uncommon yellowish green or more typical bluish green or greenish-blue (teal) to a strong purplish red akin to that of raspberries. Another Brazilian source is Malacacheta. Many Brazilian stones have a primary blue hue and are locally known as Peacock Alexandrites. A local particularly chromium rich variety is known as ‘Red Alexandrite’. This peculiar stone lacks any distinct change in colour. Other notable alexandrite occurrences include Tunduru and Manyara in Tanzania, the Indian states of Orissa and Andhra Pradesh (Araku Valley). Poorer quality stones are regularly found in the gem gravels of Sri Lanka.

Alexandrite is essentially valued for its degree of colour-change and buyers should accordingly refrain from stones with a weak colour shift. Stones with stark brown or grey modifiers that dullen the stones should equally be avoided. Stones from Russia, Tanzania and Brazil rank among the best. Fine medium-dark toned Alexandrites above 3 carats are considered very rare.

Another esteemed variety is Cat’s Eye chrysoberyl or Cymophane. It is so called for its chatoyant effect which is caused by parallel aligned hollow tubes or rutile needles inside the crystal. The most desirable Cat’s eyes combine a sharp band of light centred on the cabochon’s domed surface, a milk and honey colour effect and good translucency. Many Cat’s Eyes are destined to be set in gentlemen’s rings. Alexandrite Cat’s Eyes also exist, yet few are pretty. To be desirable such stones must feature a combination of both adequate colour change and translucency.

Faceted chrysoberyl is very popular when it appears neon greenish-yellow or lemon coloured. Beautiful stones are known from Tanzania, Sri Lanka and Orissa but are also occasionally found in Brazil and Myanmar. Of particular note is a vanadium coloured vivid bluish-green chrysoberyl that is marketed as ‘mint’ chrysoberyl. This rare colour form is only known from Tunduru in southern Tanzania.

Garnets

While most garnets remain underappreciated and inexpensive, certain colour varieties rank highly among connoisseurs. The large diversity witnessed among garnets is owed to the fact that the different members of this vast and complex mineral group readily intermix due to their interchangeable chemistry. Unique colours, a good lustre and sufficient hardness of 7 to 7.5 on the Moh’s scale enable particular garnet species to fill an important niche within the world of rare gemstones. Not surprisingly famous brands readily use Tsavorite, Mint, Mandarin Garnets and Demantoids in their jewellery pieces

Tsavorite, an intense green Grossular (after Grossularia, the gooseberry) garnet variety, is named after Tsavo (West) National Park in Kenya. A popular alternative to Emerald, its colour is owed to chromium and vanadium. Tsavorites have a firm following thanks to their crisp appearance which is due to a relatively high refractive index and good dispersion. And despite being slightly softer than Emerald at 7.25 Mohs hardness, Tsavorites are actually tougher. Discovered in 1967 by gem prospector Campbell Bridges near a place called Lemshuko in southern Kenya, Tsavorite was introduced to the public via a marketing campaign by Tiffany & Co. in 1974. Tsavorite is currently mined in small amounts in the Taita Hills of Kenya (Tsavo East) and mostly in the Merelani Hills of bordering Tanzania. Gem finds have also been reported from across the Mozambique Orogenic belt with material being recovered in the Turkana area of Kenya and Uganda as well as Tunduru in southern Tanzania and Ruangwa near the Mozambique border.  Tsavorite is also mined at Gogogogo near Tulear, Madagascar and occasionally found in Pakistan. Tsavorites from the original deposits tend to be a strongly saturated slightly yellowish green, whereas many from the Merelani deposits veer towards a slight bluish green. Yet some spectacular vibrant pure green stones unlike anything seen elsewhere have been unearthed at this deposit.  It is thought that a yellow secondary hue makes the colour vibrant, whereas blue adds richness to the hue. The market tends to prefer pure green to slightly bluish green stones. Large stones above 2ct are uncommon and specimens above 10ct are considered very rare.

The Merelani deposit is also the cradle of the so called Merelani Mint garnet which has become very popular since the 2000s. This variety has a distinct slightly bluish green mint-like colour and can best be described as a light or pastel Tsavorite of less than approximately sixty percent colour tone. The finest specimens have a highly desirable neon effect. This is due to fluorescence ‘supercharging’ the colour.

Fine Mandarin garnet or Spessartite, named after the Spessart Mountains in Germany, exemplifies the colour orange like no other gemstone. Spessartite is moreover appreciated for the highest refractive index of all orange garnets making it particularly lustrous when cut. The colour of Mandarin garnets varies from yellow-orange through the ideal vivid orange with a rare flash of red to the much less appreciated brownish or burnt orange. The best stones are either pure orange showing minor red accents or have no secondary colours at all. Stones with yellow modifiers follow whereas brown generally detracts from the beauty of this gem. Mandarin garnet first rose to the attention of the gem-world when it was uncovered at the now exhausted Kunene river-bank deposits in northern Namibia in the 1980s. Namibian Mandarin garnets, despite being mostly small in size and included, were of a mesmerizing pure orange colour.  Later a significant discovery yielding impressive and often eye clean yellowish orange, to reddish orange or amber coloured stones were discovered at Iseyin, Nigeria. Just as large and even darker reddish to brownish orange spessartites have been recovered from placer deposits at Tunduru, Tanzania. Exceptional vivid orange mandarin garnets are occasionally found at Myanmar’s Mogok gem deposits. Many of the best and pure orange ‘Fanta’ orange Mandarin garnets though come from Loliondo, Tanzania. Lesser deposits exist in Sri Lanka, Madagascar, Brazil, Pakistan and California (Ramona).

Demantoid garnets were first discovered in 1830 in Russia’s Ural Mountains whilst studying alluvial deposits along the Bobrovka river, some 110km northeast of Yekaterinburg. The gem quickly rose to become one of the favourite stones of the Tsar and was worked into astounding pieces by the court jeweller Fabergé. Demantoid is a green Andradite garnet variety that owes its name to an exceptional dispersion surpassing that of diamond. It’s colour is caused by chrome and varies from yellowish green to an uncommon vivid emerald green. Any brown modifiers seen are due to the presence of traces of iron. Demantoids are chiefly found at a few small deposits scattered across the Ural Mountains and the Green Dragon Mine at Tubussis in Namibia. The latter find produces slightly lighter but often cleaner and larger stones. Other deposits revealing the odd gem are known from Antezambato, Madagascar, the Kerman province of Iran, Afghanistan (Kunar) and Pakistan (Balochistan). However, none compare to fine Russian stones. Strong premiums are paid by collectors for specimens that contain so called ‘horsetail’ inclusions. These peculiar inclusions consist of the asbestos mineral Chrysotile radiating outward from a central chromite crystal. The largest known cut demantoids range between 12 and 15 carats, however anything polished above 2ct is considered rare.

Another commercially relevant garnet is Rhodolite. Though quite abundant, exceptional specimens of this purple-red pyrope-almandine garnet remain popular. Top quality material usually comes from either Tanzania or Sri Lanka and features an intense raspberry-red hue. A prominent find near a place called Manica in Mozambique a decade ago produced some magnificent purple Rhodolites referred to as Grape Garnets in the trade. The best stones have a distinct minor ‘bluish’ or violet hue.

Other noteworthy garnets include Malaia garnets which are composed of the garnet mineral endmembers pyrope and spessartine and come from Murakuijembe in the Umba valley but also Tunduru, both Tanzania. They feature a warm orange-pink or peach to cinnamon or pinkish-brown colour. The best stones though are devoid of brown. Yellow-green andradite-grossular garnets from Mali, simply known as Mali garnets as well as colour-change garnets are also appreciated among aficionados. Most colour-change garnets are chemically close to Malaia garnets. Better quality stones shift from a bluish-green to green in daylight to a purple-red under candlelight.  Exquisite stone are found at Bekily, Madagascar as well as Umba and Malindi in Tanzania. Small pockets have also been unearthed in Sri Lanka. Hessonite, a honey coloured Grossularite is mainly found in Sri Lanka.

Opal

Opal, the national gem of Australia, is famous for its play-of-colour. It is classed as a non-crystalline mineraloid and is composed of amorphous, hydrated silica arranged as silica spheres. Opal is associated with sedimentary basins and volcanic rocks. It has a hardness of 6 on the Mohs scale in spite of the fact that it may hold on average up to around six percent of water within its structure.

Opal is known from many sources, yet gem-quality opal ranks among the rarest gemstones in the world. It is arguably the most difficult gem to understand and appraise. Opal is valued according to its type or basic background or body colour, the dominant hue(s) of its play-of-colour, pattern, brilliance as well as size and shape.

By far the most valuable type of opal is black opal with black boulder opal being a close second. These types are closely trailed by semi-black opal which appears very dark and just slightly lighter than black opal, and dark opal which has a medium to light grey body colour. Crystal opal including jelly or water opal and lastly white or light opal follow. Black and dark opals are cherished for their opacity and contrast which enhances any play-of-colour, whereas light and crystal opals are valued for their relative transparency or translucency and are desirable as long as the stones feature sharp, brilliant colour flashes.

The different spectral colours we see in opal are determined by the actual size of the microscopic silicon dioxide spheres that constitute a particular opal and diffract the light. Small spheres create blue flashes whereas the large spheres result in a red play of colour.

The most valuable composition of spectral colours would be a multicoloured play-of-colour with a dominant red hue. Multi-coloured opal are said to contain at least three hues. An only red play-of-colour would come next on the valuation scale. Dominant orange, yellow, green, blue, indigo and violet play-of-colours and their combinations follow in descending order accordingly. Some experts believe the best multicoloured opals should display contrasting colour combinations such as red with blue.

Aside from colour a unique and striking pattern will further elevate the pedigree of a particular opal. Rare and particularly sought-after patterns will always command a premium in the market. They include Harlequin, Flagstone, Ribbon, Rolling Flash, Chinese Writing, Straw Pattern as well as Pinfire patterns.

Most importantly and irrespective of type or play-of-colour, the finest quality opal distinguishes itself by its strong brilliance and sharp colour reflections. Gem quality opal always keeps up well in indirect lighting. Some opals are afflicted by what is termed directionality, meaning that the bright reflections on the face of an opal are only visible off-axis when tilting the stone. Any obvious directionality is considered a flaw. The face of fine opal should also be devoid of any visibly dead or non-opalescent areas. This includes common opal or ‘potch’ or actual matrix such as ironstone in the case of boulder opal. In summary an opal should be judged for its overall ‘composition’ of colour, pattern, brilliance and depth.

In terms of shape regular shaped cabochons such as ovals, round or drops are preferred over random organic shapes since they easier to set in jewellery and more harmonious to the eye. When it comes to sizes, the same rules apply as with other gems: the larger, the rarer!

Opal buyers should be aware that some opal may not be particularly stable and dehydration after mining and following exposure to heat, air, sunlight or any dry environment for extended periods of time may result in the formation of cracks, so called crazing. Precious opal that shows any visible crazing must be avoided. Specialist opal dealers will sometimes temper or store opal for extended periods in order to sort out any poor material. Consequently, Opal must never fully dry out and stored accordingly!

Since Opal is somewhat fragile it should only be cleaned using warm soapy water and never be subjected to any harsh chemicals, detergents or acids. To avoid knocks or scratches opal should not be worn during manual work including gardening or dish-washing.  Opal is best mounted as pendants, earrings or cocktail-rings and is somewhat less suitable as an everyday-stone.

The best black opal comes from Lightning Ridge in New South Wales, Australia whereas fine black Boulder Opal is mined at various Queensland Opal fields. Cooper Pedy in South Australia is an abundant source of commercial White Opal.

The most important source of fire opal is Mexico. Opal is mined foremostly in the provinces of Querétaro (San Juan del Rio) and Jalisco (Magdalena). Although most Mexican Fire Opal is either yellow, orange or red and non-opalescent, some spectacular bluish Jelly (also called hydrophane or water opal) and Fire Opals showing an impressive play-of-colour have been recovered. Among Mexican Opals, the locally rare black opals are the most valued, followed by opalescent fire opal, then jelly opal and lastly the non-opalescent varieties.

A now exhausted deposit of precious white to light grey opal showing magnificent play-of-colour is located near Pedro Segundo in the Brazilian state of Piaui. A recent newcomer, producing vast amounts of light Opal, more specifically water-absorbent hydrophane or fire opal, is Ethiopia’s Welo province. A lot of material shows strong opalescence. Unfortunately, much of the material is prone to crazing.

Other notable sources of precious opal include Virgin valley, Nevada (Black Opal,) Honduras (Crystal opal) and the Island of Java, Indonesia (Jelly and Black Opal). The best-known old-world source of precious opal is Dubnik, Slovakia. A rather unusual translucent opal known as Andean Opal comes from Peru. Though it lacks any play-of-colour it possesses a remarkable green to blue colour which is caused by the presence of iron and nickel.

Buyers should always opt for solid opals. Opal doublets consisting of a sliver of opal, dark glue and backing material and triplets are basically doublets covered by a crystal (glass) or quartz dome are essentially cheap gems destined towards the commercial market or better fashion jewellery.

Tourmaline 

‘Tora mali’ or ‘stone of mixed colours’ as it is known in Sinhalese comes in all the colours of the rainbow. Tourmalines are boron silicates and form one of the most complex group of gem minerals, which is reflected in their amazing palette of colours. This diversity in appearance can be explained by differences in the essential chemistry constituting the various tourmaline species and/or varying amounts of one or more trace elements and combinations thereof. Most though not all gem tourmalines belong to the mineral Elbaite. Tourmaline is found globally in granites, pegmatites and metamorphic schists and is easily recognized by its three or six-sided prisms in its natural state. Tourmaline is also the most dichroic gemstone and directional differences in colour according to the orientation of the crystal are easily discernible. Centuries ago seafarers recognized tourmaline’s pyro- and piezoelectric properties and used the elongated prisms to pull out ashes from their smoking pipes, referring to the stone by the Dutch word ‘Asschentrekker’.

In 1989 a stunning tourmaline was unearthed at the São José da Batalha mine by garimpeiro Heitor Barbosa and named after the Brazilian province of Paraiba. It was subsequently also mined at Quintos and Mulungu in neighbouring Rio Grande do Norte. Paraiba tourmaline is fabled for its vivid neon slightly greenish blue to bluish green and dark blue to reddish purple colours which are primarily caused by the trace-element copper and varying amounts of manganese. The most appealing colour mimics a hue similar to that of turquoise and is known in the trade as ‘Caribbean Blue’. Paraiba-type tourmalines were later also discovered in Nigeria and Mozambique, yet to this day the finest gems come from Brazil. Mozambique though proved to be an especially plentiful source producing a wide array of colours hitherto unknown in cuprian tourmalines. The colours encountered range from neon yellowish-green and electric grass green through vivid lavender purple and baby blue (similar to pastel sapphires) to more typical Paraiba hues. Other magnificent tinges include hot-pink, aubergine and fuchsia. Many unheated Paraiba tourmalines have a purplish violet colour which turns into a much lighter greenish blue after heat-treatment. Unheated Paraiba’s can fetch substantial premiums. Finest Brazilian Paraiba tourmalines today rank among the most valuable gemstones in the world. Mozambique stones rank second.  They are more plentiful, cleaner and larger and commonly surpass 5 to 10ct in size but never quite reach the level of saturation and neon effect seen in original Brazilian material. Brazilian Paraiba tends to be heavily included and clean specimens above 2ct are very rare.

Pure sapphire-blue tourmalines rank among the great rarities of the gem world. Most Indicolites as connoisseurs call them feature a slightly greenish blue. However, due to a general scarcity of outstanding straight blue material these stones, which are no less beautiful; have been readily accepted by the trade. Indicolites should display bright open colours with light penetrating all crystal axes, in other words there should be no apparent ‘closing up’ of the colour as it renders the stones unattractive. This should not be confused with the ever-present pleochroism, a display of different colours in varying crystal directions due to differential light refraction, tourmaline is so well known for. Fine Indicolites are known from several deposits across Brazil, the classic one being Mutuca. More recently Bahia has been producing some fine material that closely resembles Paraiba tourmaline. However, the colour of Indicolite is usually imparted by iron and lacks the neon effect. Another notable source of superb Indicolite is the Neuschwaben mine at Erongo, Namibia which is known for its magnificent ‘lagoon’ and ‘seafoam’ tourmalines. Gemmy green-blue crystals are also regularly mined in the mountains of Pakistan and Afghanistan.

Chrome tourmaline, a form of dravite, is noted for its succulent green colour and ranges from slightly bluish green to slightly yellowish green. As the name suggests, traces of chromium but also vanadium give rise to the very best green in tourmaline. Both these colouring agents are associated with especially vibrant colours throughout the entire gem world.  The perfect hue in chromian tourmalines is a pure verdant green. Stones that feature a minor blue modifier are also well received by the trade. Specimens with a yellow secondary tinge follow, but those with brown should be avoided as it quenches the saturation. The most exquisite examples come from Landanai, Tanzania and Kenya, but also Myanmar and Brazil have produced stunning stones. Chrome tourmaline is usually fairly small and stone above 3ct are rare.

Green tourmaline, other than the aforementioned type, that features beautiful pure, open colours is similarly rare and is known from Brazil and mainly Pakistan and Afghanistan. Lighter tones are much preferred over dark tones with pure or slightly bluish green hues being the market preference. Mint tourmalines are often very beautiful and popular. A little yellow is acceptable as it may add vibrancy to the colour but any excess is deemed problematic.

Pinkish red and purplish red to red tourmalines are known as Rubellite in the trade, ruber denoting red in Latin. Fine quality material is mined at several locations in Nigeria and Brazil, Madagascar and Mozambique. Ouro Fino, a long-shut mine in Brazil produced many spectacular vivid slightly purplish red specimens. Truly fine Rubellite is supposed to neither display brown nor grey modifiers since both diminish the saturation in colour. In order to lighten its hue rubellite is quite commonly heated.

Over the last decade several small deposits of bright slightly greenish neon yellow to intense golden tourmalines have been recovered in the Lundazi district of eastern Zambia and bordering Malawi. These stones are known as Canary tourmalines in the trade.

Lastly bi- and tri-coloured tourmalines can be quite spectacular. These tourmalines show different colours either parallel to the crystal axis or perpendicular to it. The colour zoning in Bi- and tri-colour tourmalines forms due to changes in the crystal’s chemical composition during formation as the crystals grows in length and/or diameter. They are treasured by mineral collectors and gem lovers alike. In recent years there has been an enormous demand for fine specimens particularly by Chinese buyers. The rarest and most coveted multicoloured tourmalines display a colour zoning that combines pure spectral hues. 

Tanzanite 

Tanzanite is the blue gem variety of the pegmatite mineral Zoisite, a calcium aluminium silicate belonging to the epidote group of minerals. It was discovered by the miner Jumanne Ngoma in the Kiteto area of the Merelani Hills near Arusha, Tanzania. The gem was named in honour of the country by Tiffany & Co. for their 1968 marketing campaign that introduced it to the public. And to this day Tanzanite is only found in Tanzania. Tanzanite is fairly hard at 6.5 on the Mohs scale but somewhat brittle affecting durability. In its natural condition the stone is mostly yellowish brown and only rarely blue.  It’s distinct purplish to violetish blue colour is created by heating the stone to a temperature of about 380 degrees Celsius for approximately half an hour to one hour. This process leads to a charge transfer and consequently a lasting shift in colour. Connoisseurs readily pay premiums for unheated blue stones. Zoisite from the Merelani deposits also comes in a variety of other hues including pink, green and yellow, but it is the rich, slightly purplish blue that has made this gem so famous. Tanzanite’s most appealing colour is the pure blue hue possible at medium dark saturation levels. This particular deep very slightly violetish blue colour is strongly correlated with an elevated content of the trace element Vanadium, although the stones may also contain traces of Iron and Manganese. Tanzanite is further noted for a marked pleochroism and a multicolour effect and will shift colour under different lighting conditions. Its gorgeous velvety colour and the fact that is commonly clean, sizeable and relatively inexpensive enabled this gem to create a loyal following among consumers. To this day Tanzanite remains particularly popular in the US, Germany and China.

Peridot  

Peridot has been appreciated for its unique olive colour since biblical times. Gem quality peridot belongs to the magnesian end-member forsterite, a common constituent of the volcanic mineral olivine. The most fabled source of antiquity, producing many outstanding gems, is St. John’s Island, now Zabargad, off the central Red Sea coast of Egypt. Other notable present-day localities include the US state of Arizona, Hebei province in China, Åheim in Norway, as well as Central Vietnam and Afghanistan. However, the two most significant sources of large and attractive gem quality peridot are the Kaghan Valley near Naran, in Pakistan’s northern mountainous Sapat region and the area around Kyauk-Pon, Pyaung-Gaung in the Mogok gem district of Myanmar. At both places the occurrence of peridot is associated with metamorphically altered, partially serpentinised ultramafic rocks. Exceptional specimens of several thousand carats have been recovered on occasion. Out of these two, Burmese peridot is perhaps the finest in terms of colour and greatly admired for its apple-green colour. Generally, the greener the peridot, the better it is. Fine peridot should contain as little brown as possible. Very rarely Myanmar also produces phenomenal peridot cats-eyes and four-rayed star-peridots. This particular asterism effect is unlike that seen in any other gem and caused by minute square mineral inclusions which reflect the light.

©Jaensch, 2019. All Rights Reserved. Photography: Donald Woodrow Design: Kre8 Design

©Jaensch, 2019. All Rights Reserved.
Photography: Donald Woodrow
Design: Kre8 Design

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