ReCa4 O (BO3) 3 oxyborate crystals (ReCOB, where Re is a rare earth element such as Gd) were grown using the Czochralski pulling technique. The crystals belong to Cm space group and the relationships of the as-grown crystal morphology with crystallographic and physical coordinates were determined. The optimum length extensional and thickness shear vibrations of GdCOB were found for (ZYl) 40° and (YXt) 33° cuts, with electromechanical coupling factors k32 and k26, being on the order of 17.5% and 25% and piezoelectric coefficients d32 and d26 around -4.5 and 11.7 pC/N, respectively. Of particular significance is the nearly temperature independent behavior up to ≥1000 °C. Together with its high resistivity (∼5× 106 Ω cm at 1000 °C) and high mechanical quality factor (∼4000 at 1000 °C) make GdCOB and/or ReCOB crystals promising candidates for the next generation sensing applications at ultrahigh temperature (≥1000 °C). © 2008 American Institute of Physics.