Careful observation of exact chromium photoluminescence peak locations allows to make distinction or produces strong evidence in order to discriminate between natural and many synthetic emeralds. Additionally, natural emeralds can be characterized as geologically belonging to schist- or non-schist type.
The strong intensity of Raman spectrometer’s 532 nm laser beam is capable of causing noticeable photoluminescence reactions in gems which are traditionally considered as non- or weak luminescent materials. Among these PL reactions we can have the diagnostic luminescence pattern of chromium ion in beryl; Emerald or Cr- bearing aquamarine are readily identified by luminescence reaction of chromium ion occupying aluminum site in beryl structure. Some emeralds having low chromium concentration are mainly colored by vanadium, but chromium content in these stones is still high enough to cause the luminescence reaction.
Other impurities like magnesium, titanium and zinc occupying the same octahedral position in the beryl structure influence the position of detected chromium PL lines. A careful measurement of exact position of the major chromium PL lines allows to characterize the sample as belonging to one of the three major groups: synthetic emerald, natural schist type emerald and natural non-schist type emerald.
Most synthetic emeralds contain relative high Cr concentration compared to natural stones. The first clue about synthetic origin is the PL reaction’s high intensity; many synthetic emeralds saturate the spectrometer already with the minimum exposure time of GemmoRaman-532 (1 millisecond). The sample must be oriented slightly off from the laser aperture in order to get readable results. This is not a diagnostic behavior but gives a very strong indication about the synthetic origin. PL peaks for both flux and hydrothermal synthetic emeralds occur at relatively low wavelengths compared to natural stones.
Natural emeralds can be characterized as belonging to two main types of geological occurrences; schist- type and non-schist type. This method combined with inclusion studies and absorption spectroscopy is very useful in order to determine the geographical origin of emeralds.
Picture1: Photoluminescence peaks of chromium in beryl structure. All spectra have been normalized, trimmed to 4050-4300 cm-1 and background corrected. Most of the spectra are vertically shifted in order to visualize the difference between major groups. Colored bars indicate the peak location variation in each major group.
Examples of Type I (non-schist) emeralds: China (Malipo), Colombia (Chivor, La Mina Glorieta, Las Cruces, El Diamante, El Toro, La Vega de San Juan, Coscuez and Muzo), Nigeria (Jos, Gwantu), Tanzania (Sumbawanga)
Examples of Type II (schist) emeralds: Afghanistan (Panjshir Valley), Austria (Habachtal), Brazil (Carnaiba/Sotoco, Capoeirana, Itabira, Santa Teresinha), Russia (Malishevo, Perwomaisky, Mariinsky, Aulsky, Krupsky, Chitny, Tsheremshansky), Zambia (Kafubu area), Madagaskar (Mananjary, Ankadilalana), Zimbabwe (Sandawana), South Africa (Transvaal), Spain (Franqueira), Pakistan (Mingora Mines), Mozambique (Morrua district)
Inessa Moroz, Michael Roth, Micheline Boudeulle and Gerard Panczer: Raman microspectroscopy and fluorescence of emeralds from various deposits. Journal of Raman Spectroscopy 31, 485–490 (2000)