Magnetism in Gemstones
An Effective Tool for Gem Identification
© Kirk Feral
Green: Green and yellowish green Paraiba color may be due to a combination of yellow color from manganese (Mn2+ and Mn2+-Ti4+ charge transfer) and blue color from copper (Merkel & Breeding, 2009). Iron is present only in trace amounts, but it has been suggested that green color in Paraibas may also be partly due to traces of iron involved in iron to titanium (Fe2+-Ti4+) charge transfer (Laurs, et. al., 2008). Green Paraibas tend to have higher concentrations of manganese than Paraibas of other colors, and consequently they show the strongest magnetic attraction. The medium green gem pictured below shows a Drag response, and has the highest magnetic susceptibility of any Paraiba Tourmaline tested for this study.
Overview of Paraiba Tourmalines
Elbaite Tourmalines whose colors are influenced by copper impurities are referred to in the trade as Paraiba Tourmalines, cuprian Tourmalines or copper-bearing Tourmalines. Copper imparts blue color. Paraibas are rare. They were discovered in the state of Paraiba, Brazil in 1987, but since 2001 they have also been mined in Nigeria and Mozambique.
Paraiba Tourmaline Rough Crystals
(5.47ct. Total,Weak to Strong)
Blue: Copper imparts the blue color that is associated with Paraiba Tourmalines. Pure blue Paraibas are colored entirely by copper ions, which exist in concentrations too low to be magnetically detectable. Color intensities range from pale blue to deep blue. Pure blue is often modified to greenish blue or "aqua" blue by the presence of manganese (Mn2+). The presence of Mn3+ can lead to pink or red zoning within the blue body. These gems show magnetic responses that range from inert to strong depending on the concentration of manganese (Mn2+). Blue Paraiba gems that are inert (diamagnetic, SI <0) have an extremely low and magnetically undetectable concentration of manganese. An Inert response by any blue Tourmaline can be considered diagnostic for Paraiba Tourmaline colored entirely by copper. The strongly magnetic gem pictured below owes all of its blue color to copper and and all of its strong magnetism to manganese (Mn2+).
Magnetic Susceptibilities of Paraiba Colors from Least to Most Magnetic
The graph below shows the measured magnetic susceptibilities of 11 copper-bearing Tourmalines tested in this study. Each column of dots represents a color variety, presented from least magnetic to most magnetic. Each individual colored dot represents a single gem.
Light blue Tourmalines colored by iron and iron-iron charge transfer can look identical to blue Paraiba Tourmalines colored by copper (see photos below), but magnetic susceptibilities can range much lower for Paraiba Tourmalines. Blue Paraiba Tourmalines in this study showed responses ranging from Inert (Diamagnetic, SI <0) to Strong (SI 286), with magnetic variability corresponding to varying concentrations of manganese. The diamagnetic pale blue gem shown below (left) is colored entirely by copper, and the strongly magnetic pale blue gem on the right is colored entirely by iron.
Magnetic testing cannot be used to separate medium green to dark green Paraibas from medium to dark green "Verdelite" Tourmalines colored by iron, as both types can show a Drag response and have similar magnetic susceptibilities.
This concludes our discussion of magnetism in Tourmalines. For a comprehensive look at magnetism in the Garnet Group, which is even more diverse than Tourmaline, see our 12 page report on Garnets.
Magnetic testing may be useful in separating light green Tourmalines colored by copper and manganese from light green Tourmalines colored by iron, which may appear identical in color. Due to relatively high concentrations of manganese (Mn2+), all the light green copper-bearing Tourmalines tested in this study showed stronger magnetic responses and higher magnetic susceptibilities than all the light green Tourmalines colored by iron and Fe2+-Ti4+ charge transfer. Pictured below are a light green Paraiba pear and a light green cuprian Tourmaline oval with an unusual yellow hue. Both gems show high magnetic susceptibility. In the yellowish green copper-bearing Tourmaline on the right, manganese rather than copper is primarily responsible for color, and hence we do not refer to it as a true Paraiba-type Tourmaline.
© Kirk Feral 2013, All Rights Reserved. These materials may be duplicated for educational purposes only. No part of this website may be duplicated or distributed for profit, for commercial purposes, or for posting to another website without the expressed written consent of the copyright holder.
Blue Tourmaline Colored by Copper
(Mozambique, .38ct, SI <0)
Rough crystals are mostly small and found as worn pebbles rather than as well-formed crystals in matrix. Most Paraiba Tourmaline gems sold today have been heat-treated to highlight the blue or green color by eliminating any accompanying pink color. For our study, we tested 11 faceted copper-bearing Tourmalines (2 from Nigeria and 9 from Mozambique) as well as a number of rough crystals. We verified all as true copper-bearing using a UV-Vis-NIR spectrometer to detect copper content.
The amount of copper impurities in allochromatic Paraiba Tourmalines can be very small. Researchers have found that copper oxide concentrations as low as .05% by weight are common, and the highest concentration is at about 2.5%. At these levels, copper in Paraiba Tourmalines cannot be detected with a magnet, as copper oxides are 30 times less paramagnetic than iron oxides. Copper ions (Cu2+) have only one unpaired electron, and therefore induce very low magnetic susceptibilities.
Paraiba Tourmaline gem colors include blue, green and violet. Copper-bearing Tourmalines with pink, red or purple colors are not classified by gemologists as Paraiba-type Tourmalines. Paraiba Tourmalines typically have “neon” colors that are brighter than we find in Tourmalines without copper. The "neon" glow of Paraibas is probably caused by manganese (Mn2+, yellow color) brightening the color induced by copper (Cu2+, blue color). Lab-created synthetic blue Spinels, which are colored blue by cobalt, often contain traces of manganese that apparently brighten the color. A similar effect can be seen in natural bright blue Apatite gems, which derive blue color from rare earth metals, and also contain traces of manganese.
Copper-bearing vs. Iron-bearing Blue Tourmalines
Yellowish Green Paraiba
(2.05ct., Drag, SI 373)
Blue Tourmaline Colored by Iron
Afghanistan, 7.23ct, SI 230
Violet and Purple: Violet and purple colors in copper-bearing Tourmaline result from a combination of pink color from manganese (Mn3+) and blue color from copper (Cu2+). Manganese in the valence state of Mn2+ is also present in significant concentrations, as evidenced by the strong magnetic response of the violet pear shown below. Cryptic manganese (Mn2+) ions can be responsible for strong magnetic susceptibilities without contributing yellow color. Heat treatment is routinely applied to violet, purple, pink and red and copper-bearing Tourmalines to drive out pink color caused by Mn3+ (Abduriyim et. al., 2006), leaving only colorless Mn2+ and the desired blue color caused by copper. Heat changes the valence state of manganese from Mn3+ (pink) to Mn2+ (colorless, cryptic).
(2.31ct., Strong, SI 169)
Violet copper-bearing Tourmalines such as the one above qualify as true Paraibas, but technically purple copper-bearing Tourmalines such as the oval shown below are not Paraiba-type because the color is predominantly influenced by manganese rather than copper. Purple copper-bearing Tourmalines contain more Mn3+ (pink color) than do violet Paraiba gems. Magnetic susceptibility measurements indicate that concentration of Mn2+ varies widely. Magnetic responses range from Weak to Strong.
Purple Copper-bearing Tourmaline
(4.76ct., Strong, SI 174)
Purple Copper-bearing Tourmaline
(2.34ct., Strong, SI 239)
The purple gem below shows brown striated inclusions that are common in copper-bearing Tourmalines from Nigeria and Mozambique. It has been reported that these inclusions are iron oxides (Hematite) which stain the inner walls of growth tubes. Most probably, the thin layers of oxides do not contribute to detectable magnetic susceptibility.
No blue Tourmalines in this study colored by iron and iron-iron charge transfer have been found to show a Diamagnetic response as do some copper-bearing Paraibas, and no blue Tourmalines colored by copper have been found to show a Drag response as do iron-rich blue "Indicolite" Tourmalines.
Blue Paraiba Tourmaline
(Mozambique, 12.52ct, Strong, SI 286)
Pink and Red: Tourmalines that are pink or red in color and contain traces of copper are also technically not classified as Paraiba-type, although they are sometimes sold as Paraibas. The concentration of copper in these gems is necessarily very low compared to true Paraiba gems, particularly compared to blue and green gems. Any significant concentration of Cu2+ would contribute blue color, imparting a visible purple hue to pink/red gems. When Tourmalines are pure pink and pure red, color is entirely due to manganese (Mn3+) rather than copper (Cu2+). These gems are best referred to as pink Tourmalines and "Rubellite". As we showed on the previous page, pink Tourmalines and "Rubellite" are diamagnetic to moderately magnetic. The three gems pictured below were sold as Paraiba Tourmaline or copper-bearing Tourmaline, but no traces of copper can be detected in any of these gems with a spectrometer.
Light Green Paraiba
(.48ct, Strong, SI 309)
Pink Tourmaline Sold as Paraiba
(Mozambique, 2.81ct., Inert, SI <0)
"Rubellite" Sold as Paraiba
(Nigeria, 2.77ct., Weak, SI <20)
In idiochromatic gems such as blue Turquoise, copper induces weak to moderate magnetic attraction because copper within copper salts is in high concentrations (about 6% by weight). However, the strong magnetic attraction we often find in allochromatic Paraiba Tourmalines is due entirely to manganese. In Paraiba gems, manganese is frequently present in much higher concentrations than copper (up to 6% manganese oxide by weight). Iron is found only as a trace element in Paraiba Tourmalines, and such an extremely low level of iron is not magnetically detectable.
Photo by Wimon Manorotkul, courtesy of palagems.com
Some red Tourmalines can have a subtle purplish hue that is due to copper, and these can legitimately be referred to as copper-bearing Tourmalines, although they are not considered true Paraiba-type. Purplish-red Tourmalines from Mozambique and Nigeria are often heated to remove the red color completely, leaving a light blue color that is the result of copper. Although the remaining blue color may appear pale and washed-out, these previously-red Tourmalines are offered by retailers at high prices as Paraiba Tourmalines.
"Rubellite" Sold As Copper-bearing
(Mozambique, 6.68ct., Weak, SI 35)
Light Yellowish Green Cuprian Tourmaline
(1.12ct, Strong, SI 260)
Blue Paraiba Tourmaline
(Nigeria, 7.81ct, Strong, SI 234)
(California, 1.28ct., Inert, SI <0)