Golden Coral - Impregnated and Coated
Author: Gagan Choudhary
(This article was first appeared in the Gems & Jewellery, Vol. 21, No.2, pp 12-13)
In recent times, golden coral is becoming quite popular in the market place and amongst the consumers, especially in the fashion jewellery and beads. There are a number of internet shopping portals who are offering such beads and jewellery. And it is quite often to see golden coral which not only is impregnated but also coated with a thick layer of polymer / plastic. Therefore, as a result of increased popularity of golden coral, this warrants a description on the identification features along with those assisting in determining the presence of treatments. Broadly, Coral is classified as Carbonate and Horny types; carbonate types containing calcium carbonate in composition and are usually red, orange, pink, or white, while the horny type contains protein substance and are usually black, blue or golden. The horny type, Golden coral habitually belongs to the Antipatharian order, species - Stichopathes, Cirrhipathes, Leipathes (Pedersen M.C. 2004) but some may also belong to order Zoanthiniaria, species Gerardia (Pedersen M.C. 2004) and order Alcyonacea, (formerly known as Gorgonacea) (Wikipedia, 2012). Although, structural studies help to determine the species, in some cases, exact species of golden coral may not be determined as large numbers of black coral are bleached to produce golden coral.
Figure 1: This golden coral bead of antipatharian order weighing 3.44 ct, submitted to the Gem Testing Laboratory of Jaipur, India, turned out to be impregnated and coated with an artificial resin/polymer
Golden corals display features typically associated with horny corals which include curved growth layers (figure 2), concentric array of cracks appearing flakes (figure 3) surrounding a central canal (again, figure 2) that are oriented along the length of the branch. In case of beads, the central canal is usually oriented along the drill hole. These circular growth layers are also intersected by growth lines emanating from the central canal towards the surface (again, figure 2). When viewed from side i.e. the central front portion of the bead display a characteristic pimply to wavy surface with strong reflections / sheen effect (figure 4). On careful observation, the growth lines emanating from the central canal appear to cause the pimply or wavy surface at cross sectional view.
Figure 2: The encountered bead displayed features typically associated with horny corals which include curved growth layers, surrounding a central canal that was oriented along the length of the branches. Also note the growth lines emanating from the central canal intersecting the circular growth layers. These growth lines appeared to be responsible for pimply surface shown in figure 4. Magnified 32x
Figure 3: The concentric layers shown here consisted of array of cracks appearing as flakes, which is typically associated with horny corals. Magnified 64×.
It is quite common to fill the pores of coral with polymers and plastics to improve the appearance and /or durability. In some cases, the specimens are not only filled but also coated with a thick layer of polymer. Such treated samples exhibit gas bubbles present very close to the surface while some deep inside the cavities and spaces within the circular growth layers (figure 5). The presence of gas bubbles at a significant depth of the sample indicates that polymer-like substances have been filled into the cavities, but always look for the presence of coating. Presence of coating can be confirmed by observing the beads around the drill hole; a demarcation line (figure 6) will usually be seen separating coral part from a thick layer of polymer-like substance. However, in case of other cuts, determination of coating becomes lot more challenging. Presence of polymer based coating may also be indicated by the resinous surface lustre and numerous scratches on the surface (see figure 2 and 6).
Figure 4: The central front portion of the bead displayed a characteristic and distinctive pimply to wavy surface with strong reflections/sheen effect. Magnification 64×
Figure 5: Presence of gas bubbles deep inside the cavities and spaces within the circular growth layers indicated the presence of a polymer. Magnification 48×
Figure 6: A demarcation line seen near the drill hole, separating coral part from the thick layer of polymer-like substance, confirming the presence of coating. Magnification 48×
Gemmological and Spectroscopic Properties
Golden Corals show spot RI at around 1.55 and specific gravity at around 1.27. Both these values are consistent with those associated with the horny type corals, which contain protein material rather than the calcium carbonate, which significantly increases the properties. However, the values of RI and SG can be influenced by thick layers of polymer and drill hole, if present. Under ultraviolet lamp (both longwave and shortwave), golden coral may display a weak green glow, which is usually caused due to the presence of polymers. EDXRF analysis reveals the presence of bromine and iodine, as expected for any horny type coral (Hanni, 2004). In some cases or spectrometers, presence of chlorine may not be concluded as the peak overlaps with the peak of backscattered x-rays (Rh). Presence of polymers can further be concluded by FTIR spectra, which display peaks at around 2875 and 2970 cm-1.
Golden colour of coral is commonly produced by bleaching black coral and its presence cannot be determined with absolute confidence. Corals are also commonly impregnated with colourless wax, resin and polymer to improve surface lustre and durability. Due to the increased numbers of internet shopping portals selling such items, it is necessary to keep a check on the presence of such treatments, which are often ignored.
1. Hanni H.A. (2004) Black horn coral with artificial resin. Gems & Gemology, Gem News International, Vol.40, No.1, pp 78-79
2. Pedersen M.C. (2004) Gem and Ornamental Materials of Organic Origin. Elsevier Butterworth Heinemann, Oxford, UK
3. http://en.wikipedia.org/wiki/Gorgonian; last accessed June 22, 2012
All photographs and photomicrographs by Gagan Choudhary