A piezoelectric crystal element and a sensor utilizing the same are presented for use in a sensor device for identifying at least one foreign material from environment. The crystal element comprises at least one crystal resonator in the form of a inverted mesa structure, which has a membrane-like region and has a certain resonance frequency value. A surface region of the crystal resonator is modified by reactive molecules of a kind capable of interacting with the foreign material to yield a reaction product that effects a change in the resonance frequency of the crystal resonator from said certain resonance frequency value. This change is indicative of the identity and quantity of the foreign material.