Resonating sensors for use in high-pressure and high-temperature environments are provided. In one embodiment, an apparatus includes a sensor with a double-ended tuning fork piezoelectric resonator that includes a first tine and a second tine. These tines are spaced apart from one another so as to form a slot between the first and second tines. The width of the slot from the first tine to the second tine varies along the lengths of the first and second tines. Various other resonators, devices, systems, and methods are also disclosed.