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by gagan choudhary

AQUAMARINE OR BLUE BERYL?


GAGAN CHOUDHARY, FGA

Beryl, is a popular gem material found in wide range of colours and varieties – emerald (green) being the most celebrated variety, followed by aquamarine (blue), morganite (pink), heliodor (golden), goshenite (colourless), etc. In addition to aquamarine, the most admired blue coloured variety of beryl, there also exists another blue-coloured beryl, referred to as “maxixe-type” beryl. However, only few members of the trade are aware about the existence of such blue-coloured beryl, other than aquamarine. This is evident from the number of queries received at the IIGJ-Research & Laboratories Centre (formerly GJEPC-GTL Jaipur) when issued report identifies the submitted sample as “natural beryl” and not “aquamarine”. This article illustrates key differences between the two blue-coloured varieties of beryl – aquamarine and “maxixe-type” beryl.


Colour

Aquamarine typically ranges from greenish blue to blue, of low to moderate saturation with low to medium tone, while maxixe-type blue beryl is usually more intense in blue with slight violet component, similar to many tanzanites and sapphires. However, some of them appear very similar to the premium colour of aquamarine, described as “Santa Maria” colour – named after the classic aquamarine deposits of Brazil. Hence, these maxixe-type blue beryls are often presented as “Santa Maria” aquamarine in the trade and on internet marketplace.










Figure 1. Example of classic greenish blue colour of aquamarine with low to medium saturation.




















Figure 2. A ‘maxixe-type’ blue beryl displays high saturated blue, similar to sapphires.























Figure 3. Example of an aquamarine with saturated blue colour. Such saturated colours require detailed spectroscopic analyses for separation from ‘maxixe-type’ blue beryls.

Cause of Colour

Aquamarine is coloured blue due to presence of iron (Fe2+/Fe3+) on different lattice positions and channels within the beryl structure, while maxixe-type blue beryl is coloured by colour centres or defects associated with carbonate impurities. These differences are detected by absorption spectroscopy; usually a desk-model spectroscope can make such separation, however, in case of samples with low saturations, such differences are revealed by UV-Vis-NIR spectrometer.


Aquamarine typically displays peaks at ~370 and 427nm along with a broad band centered at ~825nm, while maxixe-type blue beryl displays a series of peaks in the region 500-700nm, with a dominant feature at ~690nm. Such features are typically associated with radiation-induced color centers as observed in 'Maxixe-type' beryl (see Adamo et al, "Aquamarine, Maxixe-type Beryl and Hydrothermal Synthetic Blue Beryl: Analysis and Identification", Fall 2008, G&G, pp 214-226).

Figure 4. IIGJ-RLC in routine performs UV-Vis-NIR spectroscopy to separate aquamarine and maxixe-type blue beryl. Both these gem varieties can be conclusively separated from each other as evident from the absorption spectra.

Dichroism

Maxixe-type blue beryl displays dominant blue colour when viewed down into the ‘c’ axis or length of the crystal, while aquamarine displays the opposite. This becomes one of the quickest methods to separate the two beryl types, especially in rough form.


In addition to directions mentioned above, colour component also helps to separate the two blue-coloured varieties of beryl. Aquamarine displays greenish blue and blue as two dichroic colours, while Maxixe-type beryl displays blue and colourless as two dichroic colours.

Figure 5. Dominant blue colour directions in aquamarine and maxixe-type blue beryl.