In this study, thermal properties and thermal decompositions of dehydration behaviour of gem-quality translucent blue chalcedonies, without banding or crystalline centre structure, from the Sancakaya-Eskisehir region in Turkey were studied by means of X-ray diffraction (XRD), inductively coupled plasma-atomic emission spectrometry (ICP-AES), Fourier transform infrared (FT-IR), thermoluminescence (TL), and simultaneously two thermal analyses of (DTA/TGA) spectroscopy. X-ray diffraction patterns of the blue chalcedony indicate the presence of two important chalcedonic silica phases with overlapped peaks at 4.26, 3.34, 2.28, 2.13, 1.82, 1.54, 1.38, and 137 A. During heating from the room temperature to 300 degrees C, the thermoluminescence pattern of the blue chalcedony shows a characteristic peak at 210 degrees C. This peak may be due to unusually high traces of the impurities S. Th, TI, U, and W. During heating from the room temperature to 1400 degrees C, the TGA pattern of the blue chalcedony indicates that the weight loss is due to the silanol water loss only, and that this loss occurs in a wide temperature range between about 170 and 954 degrees C. In addition, after making some corrections concerning the artefact mass gain, being due to the drift with buoyancy effect of the atmosphere in its TGA curve, the moganite-rich blue chalcedony shows a relatively lower mass loss of 0.202%. The DTA pattern of the blue chalcedony displays both endothermic and exothermic behaviours because of silica phase transformations. There are one distinctive sharp endotherm and three weaker endotherms at 806 degrees C. In addition, there is one distinctive sharp exotherm and one weaker exotherm at 1270 degrees C. (C) 2010 Elsevier B.V. All rights reserved.