Gem News International Gems & Gemology, Summer 2013, Vol. 49, No. 2

Orangy Pink Coated "Soft Coral"

Figure 1. This string of orangy pink “soft coral” beads (6–15 mm in diameter) displayed a rough surface and large pores. Photo by Gagan Choudhary.
Dyeing white coral to imitate popular colors such as red, orange, and pink is a widely known practice. But a bead strand examined at the Gem Testing Laboratory in Jaipur, India, revealed a coloring method unfamiliar to gemologists (figure 1).

The strand weighed approximately 380 ct and contained spherical beads ranging from 6 to 15 mm in diameter. These were identified as coral belonging to the species Melithaea ocracea of the order Alcyonacea (e.g., M.C. Pederson, Gem and Ornamental Materials of Organic Origin, Elsevier Butterworth-Heinemann, Oxford, 2004, pp. 192–218). Their surface showed deep pores and dull luster. The presence of pores was sufficient to identify the beads as soft coral, which is also characterized by a spongy, brittle, rough appearance.

Although their identification was straightforward, the beads’ appearance caused doubt regarding their color origin. When viewed under magnification (figure 2), distinct color concentrations were visible within the surface pores and cavities. The coloring agent was restricted to the surface, producing an enamel-like appearance in the trenches while the ridges appeared white. This evidence suggested that the beads were coated with paint rather than dyed.

Figure 2. Under magnification, distinct color concentrations are visible within the pores and cavities of this “soft coral.” The apparent use of a coloring agent only on the surface, evidenced by the enamel-like appearance within the trenches and the whiteness of the ridges, suggests these beads were coated, not dyed. Photomicrograph by Gagan Choudhary; magnified 48×.
Although the microscopic features were sufficient for identification, further tests confirmed those findings. Under UV light, the beads displayed bright orange fluorescence (more strongly under short-wave), a feature commonly associated with pink-orange dyes. No typical absorption spectra were visible under a desk-model spectroscope or recorded with a UV-Vis-NIR spectrometer. This is because the beads’ dull luster, opacity, and rough, uneven surfaces neither reflect light (in diffused reflectance mode) nor absorb it (in transmission mode). Weak and broad absorption bands were displayed at approximately 540 and 680 nm, and the presence of calcium carbonate (aragonite) was confirmed by Raman spectroscopy.

Such “soft coral” is commonly dyed red and the pores are filled with a polymer to enhance polish and luster. But this specimen’s color and luster were unusual. The lab also received similar coral strands in pink and black, suggesting this dye method can produce a variety of colors.