Luwei Fan

By Luwei Fan

Precious coral, with attractive pink-to-red color, has been used for ornamental purposes for several thousand years. According to the related science, attractive red precious corals material, are defined to the class Anthozoa, subclass Octocorallia, order Alcyonacea, suborder Scleraxonia, and family Coralliidae in zootaxy. About 30 species are discovered in Coralliidae family compared to the huge Cnidaria phylum. Corallium rubrum, Corallium japonicum, and Corallium elatius are the three main species in Coralliidae family used for jewelry material in the gem market. The purpose of this chapter is to show the nature of animals in Coralliidae family, analyze the nondestructive test methods to identify the natural species from the imitations, and discuss the origin of color and the interactions in between the organic matrix and mineral. The chapter was organized in six parts. The first part reviews the history of precious coral used for different purposes by humans and then describes exact affiliation of precious coral on zoology and taxonomy. The second part deals with the biology and formation of precious coral. The paragraph also presents the information about Coralliidae colonies’ sexual maturity, life span, growth rate, and mortality. The trade market and conservation are also summarized in this part. The gemological properties of C. rubrum, C. japonicum, and C. elatius, the main species in precious coral market, are introduced in the third part. As a consequence, an effective and nondestructive identification method to distinguish natural precious corals from their imitations was stated with Raman spectra as demonstrated in the fourth part. In the fifth part, the origin of precious coral color based on the results of Raman scattering measurements and PeakFit analysis is demonstrated. Three different excitation wavelengths (785, 633, and 514 nm) were used for the same samples at the same points. The result shows that all of the samples are colored by a mixture of pigments. Different colors are explained by different mixtures, not by a single pigment. Organic composition, even present in a small amount, plays an important role in the color of precious corals. The sixth part concludes the text by presenting what we have learned from the experimental data of microscope, SEM, TEM, and EBSD. The spatial relationship between the organic matrix and mineral components is determined by SEM observation on decalcification-treated samples. By integration of the results from nanometer to centimeter-scale detections, a hierarchical structure of precious coral is revealed. In the skeleton of precious coral, building blocks are arranged into several hierarchical levels of oriented modules. The modules in each hierarchical level assemble into larger unit that comprises the next higher level of the hierarchy. Precious coral, as a member of biomineral family, assembled skeleton as a delicate arrangement of a hierarchy of crystals with well-defined orientations under the control of organic matrix. Organic matrix works in both color pigment and architecture field for the precious coral.

Part of the book: Corals in a Changing World