[Ciclo Silicati – trigonale – AX3Y6(BO3)3Si6O18(O, OH, F)4 A = Ca, Na (K,Cs) ; X = Al,Fe,Li,Mg (Mn) ; Y = Al (Cr,Fe,V) – Hm: 7-7,5 Ps: 2,98-3,25 g/cm3 – Ir: 1,616-1,652]
Tourmalines are a group of complex aluminum borosilicates in which many metals can coexist and replace each other: they are therefore a family of mineralogical terms that crystallize in the same forms and have approximately the same physical constants. Thanks to all these possibilities of chemical substitutions they show virtually any color, and are often found in polychrome crystals.
They have higher dispersion than Topaz and Beryl, but not high enough to compete with the Fire of Diamonds and Zircon. The birefringence and pleochroism are high and are an excellent means of identification of Tourmalines, which also present the phenomena of Piezoelectricity, Pyroelectricity and are also excellent polarizers. If subjected to heating between 450-650 ° C they vary slightly in color, but it is not sure that this treatment will provide improved gems.
Frequent “icy” inclusions frayed with liquid-gaseous fluids that often give a velvety appearance, solid inclusions are generally lepidolite micas, more rarely microcrystals of garnets and zircons. Those inclusions with parallel channels can assume particular importance, which if abundant can give an interesting chalking effect, from which cabochons of CAT EYE TOURMALINE are cut.
The name TORMALINA comes from the Sinhalese “Turmali” which means “mixed”, no other gem can boast the colors that tourmaline can have. Many Russian crown jewel stones that were thought to be rubies have been reclassified as Tourmalines. The last Chinese empress Dowager Tzu Hsi loved pink tourmaline enough to buy at least a ton of it from the new Himalaya Mine in California, which is still in production today. Tourmaline is also of interest to scholars because it changes its electrical charge when heated. It becomes a polarizing crystalline magnet to attract light objects. This property was known long before science was able to explain the phenomenon. Tourmaline can have any color of the iris and combinations of 2 or 3 colors can coexist in polychrome Tourmalines. Sometimes the colors are at the different ends of the crystal. Sometimes one color is in the center of the crystal and the other or others surround it. As in the so-called “Watermelon Tourmalines” WATERMELON TOURMALINE where a central core of pink tourmaline is surrounded by a more or less yellow white band and a green outer ring. Any shade of tourmaline can be found in Brazil, especially in Minas Gerais and Bahia, the most popular are green and pink.
In 1989 tourmalines were discovered like never before. The new type of tourmaline, which soon became known as PARAIBA TOURMALINE, had an incredibly bright blue or green color, the demand and excitement for these new tourmalines quickly caused the cost of the finest stones to rise to over $ 10,000 per kt. Pink and green tourmalines are currently widely available on the market while blue ones are quite rare due to high demand. Tourmalines are often cut in Baguettes that respect their elongated and narrow crystalline habitus, they also show a strong pleochroism: the darkest color is always observed along the axis of the crystal. But for the important gems the classic Cushion cut (or Antique cut) is preferred.
Among the most beautiful tourmalines we must mention a 111 ct pink Manchurian gemstone and two 125 and 173 ct champagne-colored yellow tourmalines, exhibited at the Smithsonian Institution in Washington, as well as various Chinese or Russian sculptures and engravings present in the main collections of the world. In addition to Brazil, tourmalines are found in Tanzania, Kenya, Madagascar, Mozambique, Namibia, Afghanistan, Pakistan, Sri Lanka and in the USA, California and Maine.
A particularly beautiful variety is CHROMOTORMALINE a rare type of tourmaline from Tanzania. Tourmalines are hard and durable gems that are always in fashion but you must be very careful not to clean them with steam or to bring them close to heat sources. In addition, attention must be paid to the edges that can chip. Like the range of colors it presents, the treatment possibilities for tourmaline are many, the resulting stones are stable under normal conditions and totally safe. The most common treatments are coloring, in some cases also irradiation. More rarely, tourmalines are infused with oils to hide fractures and other cracks that reach the surface. Tourmaline has never been synthesized but is imitated in many ways from similar natural stones to artificial glass pastes.
Mineralogists distinguish various members of the mineralogical family of tourmalines:
– SCIORLO or also SCIORLITE (Borosilicate Sodium and Iron hydroxide) or Black Tourmaline
– ELBAITE (Sodium and Lithium Hydroxide Borosilicate) generally dark green almost black, but it is the gemmiferous member of tourmalines;
– DRAVITE (Borosilicate sodium, magnesium and aluminum hydroxide)
– UVITE (Borosilicate hydroxy-fluoride of Calcium Sodium Magnesium and Iron)
– BUERGUERITE (Sodium Iron and Aluminum Hydroxy-fluoride Borosilicate),
– FOITITE (Borosilicate iron and aluminum hydroxide),
– CROMODRAVITE (Borosilicate hydroxy-fluoride of Sodium Magnesium Chromium Iron and Aluminum),
– FERUVITE (Borosilicate Hydroxide of Calcium Iron Magnesium Aluminum),
– LIDDICOATITE (Sodium Lithium and Aluminum oxy-hydroxy-fluoride borosilicate),
– OLENITE (Sodium and Aluminum oxyhydroxide borosilicate),
– POVONDRAVITE (Sodium and Iron oxyhydroxide borosilicate).
From a gemological point of view, the following varieties are of some interest. All the precious gem-bearing varieties are generally referable to ELBAITE, the traditional green or polychrome tourmaline, apart from the relatively rare case of green gems cut from UVITE crystals, and to DRAVITE which provides yellow-brown gems.
– colorless variety: ACROITE of pure collector’s interest.
– red variety: RUBELLITE
– pink / purple variety: SIBELLITE
– green variety: VERDELITE
– CHROME-TOURMALINE sub-variety of Verdelite with the typical bright coloring of chrome.
– Blue variety: INDICOLITE.
– yellow variety: DRAVITE
– Polychrome variety: TWO-COLOR, TRICOLOR
– WATERMELON TOURMALINE: polychrome variety in which between the central pink part and the green border there is a thin white band
– PARAIBA TOURMALINE: Very rare tourmaline which is one of the Must of international jewelry.
PARAIBA TOURMALINE (en: Paraiba Tourmaline)
Talking about the Paraiba Tourmaline in gemology is like talking about the Phoenix. Copper and manganese (and a recent study would indicate a high concentration of traces of Gold) seem to be responsible for the colors of Paraiba.The combination of these two elements causes a wide variety of beautiful and fascinating colors in Paraiba tourmalines: emerald green, turquoise. – sky blue, sapphire blue, indigo blue, purple blue, purple red known among traders as Paraiba blue and Paraiba green and neon blue or neon green or even electric blue and electric green. In mineralogy we can define it as an ELBAITE CUPRIFERA.
These small, rare and precious tourmalines were mined in Sao José da Bathalha in the Brazilian state of Paraiba. The “Paraiba” was discovered in 1987: its discovery is due to a man and his unshakable faith, Heitor Dimas Barbosa who for years had dug with his helpers a small hill where a pegmatite emerged near the town of Sao José da Bathalha. Heitor Dimas Barbosa was not only a gem hunter he was deeply convinced that under the famous hill there was something “completely different” and his vision turned out to be correct. Since 1981 he had begun excavating in the old abandoned mines, suddenly, five and a half years after the first excavation, the first deposits of tourmaline appeared and in the fall of 1989 a handful of the finest tourmaline crystals were unearthed by one of the many dark passages. These crystals displayed colors never dreamed of before.
A high concentration of copper is responsible for the highly sought-after bright blue, turquoise and green, while red and green are caused by manganese. By heating the stones, expert cutters are able to eliminate traces of red, so that only the brilliant copper color appears.
The exceptionally vivid color of Paraiba tourmalines can only be appreciated after the gem has been cut. When it has been faceted, it exhibits a unique luster and brilliance, which makes the gem shine and shine even when the light is poor. Therefore, the color is often referred to as “Electric” “Fluorescent” or “Neon”.
The history of Paraiba should end here, but nature always has some surprises in store in fact at the beginning of 2001, suddenly, stones from Nigeria, tourmalines of a brilliant bluish-green, similar to those of Paraiba, appeared on the market. These stones, like Paraiba tourmalines, showed their true beauty after being carefully heated. Generally, their colors seem slightly more muted, but the difference is hardly discernible by anyone who is not an expert and even scientists find it difficult to define the existence of different characteristics between the Nigerian and Brazilian Paraibas. Copper-containing mines explain the similarity in chemical composition. Both types owe their color to copper and manganese. But this example of a valuable gem clearly illustrates the phenomenon of continental drift. Present-day Nigeria was originally directly connected with the north east of Brazil. So it is easy to assume that the tourmalines of Paraiba and Nigeria formed under the same geotectonic conditions, before the two continents separated.