Gemmological Characteristic and Heat Treatment of Gem-Quality Zircon from Muling, Heilongjiang Province

With the help of the conventional gemmological testing methods, this paper studied the gemmological characteristics of gem-quality zircon found in alluvia of Tertiary alkali basalt deposits associated with Cenozoic basalts from Muling, Heilongjiang Province and heat treatment of zircon was optimized. The gem-quality zircons, generally colourless to reddish brown, transparent to semitransparent, have well euhedral and tetragonal prism and tetragonal dipyramid are well developed. The colourless and yellowish zircon samples show strong orange fluorescence under short wave. Refractive indexes are generally higher than the test scope of gemmological refractometer, which lead to the samples have vitreous luster to semi adamantine luster after fine polishing. Inclusions can be divided into four categories: Primary solid inclusions, syngenetic two-phase inclusions, secondary inclusions along fractures and immateriality inclusions. After heat treatment, the colour of the zircon megacrysts have been significantly improved, which can increase the application value of zircon. The mechanism of coloration of zircon megacrysts should be closely related to lattice defect caused by radioactive elements and valence elements.