In the upcoming 6-generation (6G) revolution, the achievement of low power consumption has become a key objective in research concerning terahertz devices. As an important component of passive devices, there are very few low-loss dielectric ceramics in the terahertz range. To elucidate the mechanism of loss and promote the application of microwave dielectric ceramics for future 6G technology (covering microwave and terahertz frequencies), the terahertz responses of ANb₂O₆ (A = Zn, Co, Mn, and Ni) columbite niobates were studied. The influences of magnetic loss on the Q×f values in the microwave range with different transition metal ions in the A-site were reasonably analyzed. Moreover, due to the weakened magnetic relaxation properties in the terahertz range, the samples all exhibited low loss and approximate transparency, especially for MnNb₂O₆ and NiNb₂O₆ (tanδ < 0.01 and absorption coefficient < 10 cm⁻¹ below 1.2 THz), which subverted the definition of traditional low-loss microwave dielectric ceramics. Ultimately, based on Mie theory, we designed a prototypical broadband metamaterial reflector to validate the applicability of the ANb₂O₆ system in the terahertz band, which is highly important for the development of terahertz ceramic-based passive devices.