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Incolor Reading|The neon light blue "Paraiba"

2020-11-18 Source:GDGJE

"Paraiba" is a tourmaline with bright blue and green tones. It is deeply loved by people because of its unique and beautiful neon electric blue color.

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By Claudio Milisenda, DSEF German Gem Lab, Idar-Oberstein, Germany

Tourmaline surpasses all other gemstones by its wide range of colors. The group includes a number of complex boron-aluminum silicate minerals with structures that allow the incorporation of almost any chemical element.

At the end of the 1980s, unusual vivid blue and green elbaite tourmalines from the Paraíba State in northeastern Brazil generated major interest in the gem trade due to the discovery of new colors that had never been seen before in this gemstone species.

It was the first time that the trace element copper, together with variable amounts of manganese, was reported as a coloring agent in this species of gemstone. To separate these unique tourmalines from all other gem tourmalines, they were commonly referred to as “Paraíba tourmaline.” In just a few decades, they have developed into one of the most sought-after gems in the world.

01 The mine of Paraiba

Paraiba tourmalines do not only occur in the now legendary Batalha pegmatites, but are also reported to come from two other areas in neighboring Rio Grande do Norte State. Two mines are known: Mulungu and Altos dos Quintos. The former is also known as Boqueirão, Boqueirãozinho, Capoeira or CDM .

In these mining areas, tourmaline mainly appears in the form of crystal fragments in pegmatite, and it is difficult to find well-developed large crystals. The weight of the rough is usually less than 5 carats, and the weight of the faceted product is less than 1 carat. Occasionally, 4 to 10 carats of crystals will be found. In extremely rare cases, rough stones weighing 20 carats can be found.

The tourmaline-bearing pegmatites occur in metamorphic rocks of the Brasiliano orogeny, a largescale mountain building process that occurred approximately 650 to 500 million years ago, which also affected large parts of the African continent.

The material from the Edoukou mine was commonly violet in color and showed a vivid aquamarine-type blue after heat treatment. For the Nigerian specimens, in addition to manganese, copper was identified as a colorcausing agent.

Unlike Brazilian tourmalines, the Nigerian specimens originate from secondary deposits, hence rough materials are usually water-worn. In addition to the predominant violet hues, specimens with natural blue, blue-green, purplepink colors as well as multicolor were observed. Vivid turquoise blue and emerald colored specimens could be obtained through appropriate heat treatment.

02 The Cause of Color

Blue and green hues in tourmaline are usually caused by traces of iron. Chromium and vanadium could also be responsible for the green color of a tourmaline. However, for the first time, traces of copper together with manganese were found to cause the stunning colors seen in Paraiba tourmalines.
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Polarized absorption spectra of a blue cuprian elbaite as well as the calculated position in the CIELAB color circle.
(Image: Tom Stephan)

Picture above shows representative polarized absorption spectra of a blue copper- and manganese-bearing elbaite tourmaline from Mozambique. The combination of the distinct absorption bands of divalent copper with maxima at 700 nm and 900 nm causes a strong absorption of the whole red part of the visible spectrum. The transmission window in the green-blue-violet part is modified by absorption bands of trivalent manganese, which finally are responsible for local transmission maxima in the blue, blue-green and blueviolet.

Heat treatment can reduce Mn 3+ to Mn 2+ and modify the resulting color. The presence of Fe 2+ bands can have an additional effect on the color, particularly when the copper concentration is low. In green specimens, Mn 3+ bands are lacking . Here, a Mn 2+ -Ti 4+ intervalence charge transfer band starting at around 500 nm towards the UV-range is particularly obvious in the absorption spectrum of the ordinary ray. This results in a transmission maximum at around 550 nm, i.e. in the green-yellow spectral range. The extraordinary ray shows a wider transmission maximum, including the blue-violet spectral range.

Heat treatment will reduce Mn3+ absorption. This happens at relatively moderate temperatures in the range between approximately 500°C and 600°C and is, in most cases, not detectable. The degree of color change strongly depends on the trace element content and the absorption spectra of the precursor material. Measured absorption spectra and calculated copper to manganese ratios indicate whether a tourmaline is heat-treatable or not. Stones without copper or with very low copper to manganese ratios will turn colorless or near colorless after being heated.

Recently, an increasing number of clarity-enhanced stones have appeared on the market. This is because the occurrences tend to produce, if at all, more and more heavily included and fractured material. Impregnation with colorless organic substances will enhance the outer appearance. Note, however, that foreign substances could also accidentally penetrate into fissures and fractures.


03 Paraiba Imitation

Imitations and substitutes have been reported since the first appearance of Paraíba tourmaline. Among the list of imitations, there are, for example, yellowish-green fluorite, amethyst, apatite, composite stones, irradiated topaz, synthetic greenish-blue beryl, “water-worn” synthetic corundum, as well as various colored artificial glasses.

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Two apatites (left and right) with a Paraíba tourmaline weighing 1.79 ct.
(Photo: Stefan Koch)

Additional examples of substitutes worth mentioning among also presented as Paraíba tourmalines are greenish-blue and bluishgreen apatites, yellowish-green tsavolite as well as quartz doublets.

According to the nomenclature of the World Jewellery Confederation (CIBJO), green to blue tourmalines colored by copper and manganese may also be called “Paraíba tourmaline” in the trade, regardless of their origin.

Violet, purple and pink colored cuprian tourmalines, however, are not described as Paraíba tourmaline. A rather important argument here is that, in many of these cuprian tourmalines, the copper content is so low that they appear almost colorless after heat treatment. The term “Paraíba Tourmaline” would hence be misleading. They remain interesting stones, as they are offered as evidently untreated cuprian tourmalines.


Conclusions

Although known for only a few decades, Paraíba tourmalines belong today to the most valuable and sought-after stones. Extremely rare and fine specimens with a high carat weight originating from the locality of São José de Batalha in the Brazilian State of Paraíba can reach seven-digit carat prices. The production in Brazil is sporadic and focused on melée and matrix stones obtained from the overburden. Attempts at expanding underground mining into greater depths have mostly been futile.

The vivid blue and green colors are caused by a unique combination of the trace elements copper with varying amounts of manganese. New discoveries in Africa have led to considerable debates regarding the appropriate use of the term “Paraíba Tourmaline.” Concerns that this is going to erode the value and importance of the original Paraiba tourmaline or lead to the loss of the importance of Paraiba as an origin have been in vain. Instead, prices continue to climb.


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The content is organized from 《InColor》2020 Summer.

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