Pink to red coral can be readily identified by microscopic examination of the characteristic grooves or channels that the coral polyps used to transport nutrients. However, after positive identification two important questions still remain. Is it natural color or has the material been bleached & dyed? Is it legal species (not a member of Stylasteridae family which is protected by CITES) ? GemmoRaman-532 is cabable for answering both of these important questions.
Acetone and cotton swab has been the traditional method for detecting dyed coral. This technique is unfortunately somewhat destructive, removing some of the color from the surface of dyed samples. Raman/PL spectroscopy offers fast non-destructive method for detecting naturally colored coral.
Raman/PL spectra of red coral typically exhibit a mixture of coral matrix (calcium carbonate) and organic unmethylated polyenes (polyacetylenes) or methylated polyenes (carotenoids) responsible for the color. Calcium carbonate may exist in the form of calcite or aragonite. Both naturally colored and dyed corals may exhibit Raman features of these minerals.
Figure 1: Raman/PL spectra of calcite (Iceland spar) and pink coral showing the peaks of both calcite and polyene compounds.
Naturally colored red coral can be easily identified by the presence of peaks associated to organic pigments. Most important peaks are 1520 cm-1 & 1132 cm-1 and smaller ones at 1297 cm-1 & 1020 cm-1. Broad scan photoluminescence spectrum reveals series of additional Raman peaks caused by the same pigments. Overall appearance of spectrum is characteristic for coral and can only be mistaken for spectra of pink to red pearls (for example conch pearl) . Intensity of the peaks is directly correlated to color saturation of the sample. Dyed coral does not show these peaks although sometimes tiny peaks at differing locations caused by dye substance can be observed riding on the top of broader photoluminescence band.
Figure 2: Raman/PL (GemmoRaman-532) spectra of naturally colored & dyed pink to red Corallium Rubrum
Endangered Stylasteridae family protected by CITES
Protected Stylaster corals can be differentiated from more common Corallium genus corals by Raman/PL spectroscopy. Stylasteridae is the only red coral family used in jewelry in which color is actually caused by carotenoids instead of polyenes. Additionally, Stylaster’s skeleton is composed of aragonite instead of more common calcite. Most important difference is this: 1132cm-1 peak of polyenes (Corallium) is shifted about 25cm-1 to 1155cm-1 in carotenoids (Stylasteridae). In order to verify the sample does not belong to Stylasteridae family one must pay attention to the location of this peak. Additionally, a small difference may be seen between calcite and aragonite Raman fingerprints. While aragonite (Stylasteridae) has a small peak or peaks between 702 and 706cm-1 more common calcitic corals exhibit a peak at 714cm-1. A peak or shoulder at 1085-1088 cm-1 is best to be ignored. Both calcite and aragonite exhibits a Raman peak in this area.
Note! Following information about conch pearl has no literature references and is purely based on our own findings with limited amount of samples. More work has to be done in order to verify the data.
Conch pearl’s Raman/PL spectrum looks very similar to red coral, but apparently most of the peaks are shifted slightly towards blue. Additionally, since conch pearl is mainly composed of aragonite, it is possible to see a Raman peak of aragonite at 703cm-1 instead of calcite peak at 714 cm-1. Differentiation between conch pearl and coral must be confirmed with microscopic examination of growth structures.
Figure 3: Raman/PL spectra (GemmoRaman-532) of naturally colored Corallum rubrum vs. pink conch pearl. Conch spectra are shifted upwards for convenience.
Smith C.P., McClure S., Eaton-Magaña S., Kondo D. (2007) Pink- to-red coral: A guide to determining origin of color. Gems&Gemology Spring 2007, Vol. 43, No. 1, pp. 4–15.
Stefanos Karampelas, Emmanuel Fritsch, Benjamin Rondeau, Aude Andouche, and Bernard Métivier: Identification of the endangered pink-to-red Stylaster corals by Raman spectroscopy. Gems&Gemology Spring 2009, Vol. 45, No. 1, pp 48-52