Abstract: Black coral is one of the main varieties of horny coral，and in the market Cirrhipathes are always used as the substitute for black corals.In order to find out the characteristic differences between black corals and Cirrhipathes，scanning electron microscope and infrared spectrometer were used and cooperated with the conventional gemmological methods.
The testing results show that black corals and Cirrhipathes have similar characteristics in appearance and growth and behave similar physical optical properties. The main colour of black corals and Cirrhipathes is brown to black, which is distributed unevenly, while the main colour of bleached Cirrhipathes is golden yellow, All the samples are opaque and waxy luster, with the refractive index of 1.56. The density of black coral is about 1.291-1.298 g/cm³ and the density of Cirrhipathes and bleached Cirrhipathes is about 1.350-1.390 g/cm,1.204-1.240 g/cm³respectively. All the samples behave inert under ultraviolet lamp.The hardness of them is about 2.5-3.0. The fractures of black corals and Cirrhipathes show uneven-stagger shape.
Microsection observation shows that black corals and Cirrhipathes both have the concentric circle structure with the radial structure. The growth channels of black corals are accompanied with a plurality of inconspicuous stomatal structure. The growth channel of Cirrhipathes has a single clear structure of stomatal, The papular protrusions on black coral surface have a sparser distribution, whereas the papular protrusions on Cirrhipathes sur faceare concentrated.
Scanning electron microscope observation reveals that black corals are mainly composed of a large number of close cuticle arrangement, The width of cuticle is about 0.3-0.5 μm and has obvious direction and stratification. The upper diameter of the papular protrusion adhering on the side surface is about 20-30 μm. But Cirrhipathes samples do not have a clear layered structure, but closely overlapped and aggregated by the tiny scales, The upper diameter of the papular protrusion adhering on the side surface is about 100-200 μm, and on the upper side there adheres the second level protrusion, whose diameter is about 10 μm.The side surface of the first level protrusion is associated with scalelike structure, whose length is about 1-5 μm.
FT-MIR testing result indicates that the organic matter composition of black corals,Cirrhipathes and bleached Cirrhipathes are basically the same, and they practically do not contain calcite or aragonite. Surface micromorphology and growth microstructure features are effectively helpful in distinguishing black corals and Cirrhipathes.