Research On Gemological And Mineralogical Characteristics Of "Coral Jade"

"Coral jade"is a kind of coral fossils formed by the calcareous skeleton secreted by coral polyps after a long geological process and fossilization.In this paper,the gemological characteristics,mineral composition,elemental content,structural structure,chromogenic mechanism of“coral jade”were studied by petrographic observation,conventional gemological test,IR spectrum,UV-VIS,Laser Raman Spectrum Analysis,EPMA,etc.The color of"coral jade"is mainly white,red and yellow with nonuniform distribution,The"coral jade"is subtranslucent to opaque.Ultraviolet florescence is inert.The refractive index of"coral jade"is 1.48-1.54.The hardness range of"coral jade"is 3-6.5.The relative density range of"coral jade"is 2.54-2.72.At present,the common"coral jade"in the market is mainly of calcium and silicon.The calcareous"coral jade"mainly presents phanerocrystalline aggregate with the main mineral composition of calcite,while the siliceous"coral jade"is mainly composed of quartz and mostly presents cryptocrystalline aggregates.According to the morphology of coral skeleton,the structure of"coral jade"can be divided into radial structure,concentric radial structure,phoenix tail structure,etc.Polarizing microscope shows that the biological bone part of calcareous"coral jade"is composed of carbonate microcrystalline and the matrix between the bone is mainly composed of phanerocrystalline calcite,siliceous"coral jade"presents the growth rule of finely short fiber-radial fiber-long fiber-phanerocrystalline granular quartz from skeleton to matrix.The results of electron probe indicated that the crystal degree of quartz is probably related to the content of AI element which is more inclined to generate phanerocrystalline quartz with the decrease of AI content.The relative splitting of the infrared spectrum near the peak value of 800cm^-11 indicates that the internal particle arrangement is highly ordered.Different structures of"coral jade"have different genesis,the origin of the radial coral skeleton structure belongs to filling action.The origin of concentric radial structure and the phoenix tail structures is metasomatism.The color genesis of"coral jade"is rich in color with nonuniform tone.Infrared spectrum and laser raman spectrum analysis show that Mn²⁺,Fe²⁺and Fe³⁺are the main reasons for the color of"coral jade".Hematite and goethite may be the main mineral of the red and yellow particles.Organic matter is not the main cause of color of"coral jade".