A gem variety of zoisite with a blue to blue-violet colour.
Natural and heat-treated tanzanite is strongly pleochroic
Note that a significant percentage of tanzanite crystals on the market have been heat-treated (400-500°C) to produce or enhance this colour, many are originally brown or pale grey . A red tint seen looking down the c-axis is a good indication that the crystal colouration is natural. Note also that tanzanion is used as a synthetic simulant for tanzanite.
**************************
Tanzanite is relatively new to the colored stone galaxy. As the most common story of the tanzanite mining boom goes, in 1967 a Masai tribesman stumbled upon a cluster of highly transparent, intense blue crystals weathering out of the earth in Merelani, an area of northern Tanzania. He alerted a local fortune hunter named Manuel d’Souza, who quickly registered four mining claims.
D’Souza hoped that he’d been shown a new sapphire deposit. Instead, the deposit contained one of the newest of the world’s gems.
Within a short time, 90 more claims appeared in the same 20-square-mile area. No one was quite sure what the beautiful crystals were, but everyone wanted to lay claim to the profits they were certain to produce. The new gem would eventually be known as tanzanite, and it would, at times, rival the Big 3 in popularity.
Tiffany & Company recognized its potential as an international seller and made a deal to become its main distributor. Tiffany named the gem after the country it came from, and promoted it with a big publicity campaign in 1968. Almost overnight, tanzanite was popular with leading jewelry designers and other gem professionals, as well as with customers who had an eye for beautiful and unusual gems.
The instant popularity of this transparent blue to violet to purple gem was tied to its vivid color, high clarity, and potential for large cut stones.
Zoisite (tanzanite) is 6 to 7 on the Mohs scale. It has fair to poor toughness and a property called cleavage, which is a tendency to break when struck. While tanzanite is not as hard or as tough as a stone like sapphire, it still can be worn in all types of jewelry with proper precautions against rough wearing or hard blows.
Stability
Tanzanite is stable under normal wearing conditions, which means it’s resistant to the effects of heat, light, and common chemicals. Even so, tanzanite should not be subjected to very high temperatures or sudden temperature changes. It can be attacked by exposure to hydrochloric and hydrofluoric acids.
Cleaning
Warm, soapy water is always safe. Ultrasonic and steam cleaners are never recommended for tanzanite
Treatment and durability considerations
Tanzanite is routinely heat-treated to modify its color from a predominately brownish appearance to a predominately blue to purple appearance. The treatment is stable with no additional durability concerns.
There have been instances where tanzanite was coated to modify the color. Although rarely done, the coatings are not permanent and can be scratched or damaged through normal wear.
Like other transparent gemstones, tanzanite can be fracture filled to improve apparent clarity, although this is an uncommon tanzanite treatment method. The materials used to fill fractures in gemstones can alter in appearance over time.
*****************************
Tanzanite is the blue/violet variety of the mineral zoisite (a calcium aluminium hydroxyl Sorosilicate) belonging to the epidote group. It was discovered by a Tanzanian Jumanne Mhero Ngoma in the Mererani Hills of Manyara Region in Northern Tanzania in 1967, near the city of Arusha and Mount Kilimanjaro. Tanzanite is a rare gemstone with similar color as the sapphire after undergoing artificial heat treatment to form a deep blue coloration. Naturally formed tanzanite is extremely rare[3] and is endemic only to the Mererani Hills.[4]
Tanzanite is noted for its remarkably strong trichroism, appearing alternately sapphire blue, violet and burgundy depending on crystal orientation.[5] Tanzanite can also appear differently when viewed under alternate lighting conditions. The blues appear more evident when subjected to fluorescent light and the violet hues can be seen readily when viewed under incandescent illumination. Tanzanite is usually a reddish brown in its rough state, requiring heat treatment to bring out the blue violet of the stone.[6]
The mineral was named by Tiffany & Co. after Tanzania, the country in which it was discovered. In 2002, the American Gem Trade Association chose Tanzanite as a December birthstone, the first change to their birthstone list since 1912.[7]
Commercial history
The craft work on tanzanite.
Manuel d'Souza, a tailor and part-time gold prospector living in Arusha (Tanzania), found transparent fragments of vivid blue and blue-purple gem crystals on a ridge near Mererani, some 40 km (25 mi) southeast of Arusha.[8] He assumed that the mineral was olivine (peridot) but after soon realizing it wasn't, he concluded it was "dumortierite" (a blue non-gem mineral). Shortly thereafter, the stones were shown to John Saul, a Nairobi-based consulting geologist and gemstone wholesaler who was then mining aquamarine in the region around Mount Kenya. Saul, with a Ph.D. from M.I.T., who later discovered the famous ruby deposits in the Tsavo area of Kenya, eliminated dumortierite and cordierite as possibilities, and sent samples to his father, Hyman Saul, vice president at Saks Fifth Avenue in New York. Hyman Saul brought the samples across the street to the Gemological Institute of America who correctly identified the new gem as a variety of the mineral zoisite. Correct identification was also made by mineralogists at Harvard University, the British Museum, and Heidelberg University, but the very first person to get the identification right was Ian McCloud, a Tanzanian government geologist based in Dodoma.[9][10]
Scientifically called "blue zoisite", the gemstone was renamed as tanzanite by Tiffany & Co., who wanted to capitalize on the rarity and single location of the gem, and thought that "blue zoisite" (which might be pronounced like "blue suicide") wouldn't sell well.[11] Tiffany's original campaign advertised that tanzanite could now be found in two places: "in Tanzania and at Tiffany's".
From 1967, an estimated two million carats of tanzanite were mined in Tanzania before the mines were nationalized by the Tanzanian government in 1971.
Multicolored Tanzanite Crystal
Untreated Tanzanite Gemstone
Hologram of a Trichroic Tanzanite Specimen
A rough sample of tanzanite.
Recent tanzanite mining developments
In 1990, the Tanzanian government split the Tanzanite mines into four sections: Blocks A, B, C and D. Blocks A and C were awarded to large operators, while Blocks B and D were reserved for the local miners. In 2005 the government renewed the lease of Block C mine to Tanzanite One, who paid 40 million dollars for their lease and mining license.[citation needed]
In June 2003, the Tanzanian government introduced legislation banning the export of unprocessed tanzanite to India (like many gemstones, most tanzanite is cut in Jaipur). The ban has been rationalized as an attempt to spur development of local processing facilities, thereby boosting the economy and recouping profits. This ban was phased in over a two-year period, until which time only stones over 0.5 grams were affected.[citation needed]In 2010, the government of Tanzania banned the export of rough stones weighing more than one gram.[12]
Tanzanite One Mining Ltd is owned by Richland Resources, but a 2010 law in Tanzania required them to cede 50% ownership of their mining license to the Tanzanian State Mining Company (Stamico). Production in 2011 amounted to 2.4 million carats, earning them $24 million.[13]
The world's largest rough tanzanite was a 16,839 carat (3.38 kg, or 7.46 lb) stone mined by TanzaniteOne in 2005.[14]
Factors affecting value: grading
There is no universally accepted method of grading colored gemstones. TanzaniteOne, a major commercial player in the tanzanite market, through its non-profit subsidiary, The Tanzanite Foundation,[15] has introduced its own color-grading system.[16] The new system's color-grading scales divide tanzanite colors into a range of hues, between bluish-violet and violetish-blue.
The normal primary and secondary hues in tanzanite are blue and violet. Untreated tanzanite is a trichroic gemstone, meaning that light that enters this anisotropic crystal gets refracted on different paths, with different color absorption on each of the three optical axes. As a result of this phenomenon, a multitude of colors have been observed in various specimens: shades of purple, violet, blue, cyan, green, yellow, brown and red. After heating, tanzanite becomes dichroic. The dichroic colors range from violet through bluish-violet and violetish-blue to blue.[17]
Clarity grading in colored gemstones is based on the eye-clean standard, that is, a gem is considered flawless if no inclusions are visible with the unaided eye (assuming 20/20 vision).[17] The Gemological Institute of America classifies Tanzanite as a Type I gemstone, meaning it is normally eye-clean. Gems with eye-visible inclusions will be traded at deep discounts.
Heat treatment
Tanzanite forms as a brownish crystal and is trichroic - which means it shows three colors - brown, blue and violet concurrently. Heating - either occurring underground naturally by metamorphic occurrences or above ground by man in a furnace removes the brown or burgundy color component to produce a stronger purplish-blue color and makes the stone "dichroic" which means it only reflects blue and violet.[18] Rarely, gem-quality tanzanite will heat to a green primary hue, most always accompanied by a blue or violet secondary hue. These green tanzanite have some meaningful value in the collector market, but are seldom of interest to commercial buyers.[19]
When heat treated in a furnace, the temperature is usually between 370-390 degrees Celsius for 30 minutes. The stones should not have any cracks or bubbles, as they could shatter or the cracks/ bubble could increase in size during furnace heating.[20]
Some stones found close to the surface in the early days of the discovery (in an area now called D block) were gem-quality blue without the need for heat treatment - probably the result of a wild fire in the area which heated the stones underground. This gave rise to the idea that "D block" stone were more desirable than tanzanite found in other areas of the small tanzanite mining area.
Since heat treatment is universal, it has no effect on price, and finished gems are assumed to be heat-treated. Gemological Institute of America states that the source of heating is gemologically undetectable, but is assumed because of its prevalence.[21]
Tanzanite may be subjected to other forms of treatment as well. Recently, coated Tanzanites were discovered and tested by the AGTA and AGL laboratories.[22] A thin layer containing cobalt, determined by X-ray fluorescence, had been applied to improve the color. It was noted that "coatings in particular are not considered permanent", and in the United States are required to be disclosed at the point of sale.
**************************************
Eine Schmuckstein-Variatät des Minerals Zoisit mit blauer bis blau-violetter Farbe. Beachten Sie, dass ein beträchtlicher Prozentsatz der auf dem Markt befindlichen Tansanitkristalle einer Hitzebehandlung unterzogen wurden (400-500°C), um diese Farbe zu erreichen oder zu intensivieren. Viele davon waren ursprünglich braun oder blassgrau. Ein roter Farbstich beim Blick entlang der c-Achse ist ein guter Indikator dafür, dass die Kristallfärbung natürlich ist. Beachten Sie außerdem, dass Tansanion als synthetische Nachahmung von Tansanit angeboten wird. Natürlicher und hitzebehandelter Tansanit ist deutlich pleochroisch.
************
Tansanit
Tansanit |
---|
|
Allgemeines und Klassifikation |
---|
Chemische Formel | Ca2Al3(SiO4)3OH |
Mineralklasse (und ggf. Abteilung) | siehe Zoisit |
|
Ähnliche Minerale | Amethyst, Cordierit, Lazulith, Saphir, Spinell |
Kristallographische Daten |
---|
Kristallsystem | orthorhombisch |
Kristallklasse; Symbol | orthorhombisch-dipyramidal; 2/m 2/m 2/m |
Physikalische Eigenschaften |
---|
Mohshärte | 6,5 bis 7 |
Dichte (g/cm3) | 3,2 bis 3,4 |
Spaltbarkeit | vollkommen nach {100} |
Bruch; Tenazität | uneben, spröde |
Farbe | purpur, saphirblau, rotviolett, grüngelb |
Strichfarbe | weiß |
Transparenz | durchsichtig |
Glanz | Glasglanz, Perlmuttglanz auf Spaltflächen[1] |
Kristalloptik |
---|
Brechungsindizes | nα = 1,691 nβ = 1,692 nγ = 1,700[1] |
Doppelbrechung | δ = 0,009[2] |
Optischer Charakter | zweiachsig positiv[2] |
Pleochroismus | sehr stark (dunkelblau, grün-gelb und rot-violett) |
Weitere Eigenschaften |
---|
Chemisches Verhalten | nicht säurebeständig |
Besondere Merkmale | hitzebeständig bis 400 Grad C |
Tansanit ist eine blaue Varietät des grauen bis grünlichen Minerals Zoisit. Er wird wie die rosarote Varietät Thulit ausschließlich als Schmuckstein verwendet.
Etymologie und Geschichte
Die ersten Funde des transparenten, purpur-bläulich bis lila farbenen Tansanit gab es 1967 im Norden Tansanias. Bekannt wurde der Stein erst durch den New Yorker Juwelier Tiffany. Dieser benannte ihn nach seinem Herkunftsort Tansanit, da die Bezeichnung Zoisit ihn zu sehr an das englische Wort für Selbstmord ("suicide") erinnerte.
Bildung und Fundorte
Tansanit bildet sich vorwiegend aus hydrothermalen Lösungen in Gängen und Klüften von Gneisen.
Das einzige abbauwürdige und kommerziell genutzte Vorkommen an hochwertigen Tansanit-Kristallen ist auch heute noch in den Gilewy Hills bei Arusha in Tansania.[1][2] Die aktuellen Funde erreichen allerdings nur mehr selten die herausragende Qualität früherer Jahre.[3]
Aus der Region Hindukusch in Pakistan wurden zwar ebenfalls Tansanitfunde gemeldet, allerdings erreichen diese Steine weder die Farbgüte noch die Reinheit derjenigen aus Tansania und lassen sich auch durch Brennen nicht in die begehrte blauviolette Farbe überführen.[3][4]
Verwendung als Schmuckstein
Normalerweise tritt Tansanit in Größen zwischen 0,5 und 10 Karat auf. Selten finden sich auch größere Kristalle und Tansanit-Katzenaugen. Als bisher größter bekannter, unbeschädigter und unbehandelter Tansanitkristall gilt der 22 Zentimeter hohe und 11.000 Karat schwere „Kilimandscharo“.[5]
Sehr ausgeprägt ist die Mehrfarbigkeit (Pleochroismus) des Tansanits in den Farben dunkelblau, grün-gelb und rot-violett. Je größer der Stein, umso intensiver die Farbe. Dank seiner ungewöhnlichen Ausstrahlung und mit Hilfe des New Yorker Juweliers Tiffany avancierte er rasch zu einem der begehrtesten Edelsteine der Welt.[6]
Wegen seiner hohen Transparenz ist das Mineral als Schmuckstein sehr beliebt und erzielt hohe Preise. Aufgrund seiner ausgeprägten Spaltbarkeit nach einer Richtung reagiert Tansanit nicht nur empfindlich auf Druckbelastungen wie sie bei Schleif- und Fassarbeiten auftreten, sondern auch auf ungleichmäßige Wärmebelastungen bzw. rasche Temperaturwechsel. Bereits das Abnehmen des Reparaturgegenstandes von der Lötkohle oder das Auflegen des Stücks am Lötblech oder Brettstock zur Abkühlung kann zu Rissen im Stein führen.[7] Ebenso wenig verträgt der Edelstein eine Reinigung im Ultraschallgerät. Auch auf Säuren bzw. Säuregemische reagiert er empfindlich.
Da der Tansanit eine für Edelsteine relativ geringe Mohshärte von 6,5 bis 7 aufweist, die unter der von Quarz (7) liegt, muss auch beim Tragen von Tansanitschmuck die Empfindlichkeit des Steins berücksichtigt werden. Bei grober Behandlung können sonst unter Umständen schnell Kratzer entstehen.
Manipulationen und Imitationen
Ein großer Teil der im Handel befindlichen Tansanite wird durch Brennen bei etwa 400 bis 500 °C[2] aus graubraunen bis gelbbraunen Zoisiten hergestellt. Das Brennen verstärkt zudem die blaue Farbe schwach gefärbter Steine. Tansanit wird aber auch durch gefärbtes Glas (billige Variante) oder Dubletten imitiert.
Synthetischer Tansanit ist bisher nicht bekannt. Bei dem auf Mineralbörsen gelegentlich angebotenen "synthetischen Tansanit" handelt es sich meist um synthetischen Forsterit.[8]
Siehe auch
Literatur
- Walter Schumann: Edelsteine und Schmucksteine. Alle Arten und Varietäten der Welt. 1600 Einzelstücke. 13. überarbeitete und erweiterte Auflage. BLV Verlags-GmbH., München u. a. 2002, ISBN 3-405-16332-3, S. 176.
- Bernhard Bruder: Geschönte Steine. Neue Erde Verlag, 2005, ISBN 3-89060-025-5, S. 101.