Resonant devices:

While the resonant circuits consist of coils and capacitors in the series or parallel, there are other types of hardware which exhibit resonance. Some of these are stated below.

Crystals:

Pieces of quartz, when cut into thin wafers and applies some voltages, will vibrate at the high frequencies. Due to the physical dimensions of such type of a crystal, these vibrations occur at the precise frequency fo, and at whole number multiples of f_o. Which multiples, 2f_o, 3f_o, 4f_o, and so on, are called as harmonics. The frequency f_o is known as fundamental frequency of crystal. Quartz crystals can be made to behave like LC circuits in the electronic devices. A crystal exhibits impedance which varies with frequency.  The reactance is zero at fo and harmonic frequencies.

Cavities:

Lengths of metal tubing cut down to specific dimensions, exhibit resonance at high, ultra high, and microwave frequencies.  They work in the same way as the musical instruments resonate with the sound waves. But waves are electromagnetic,  rather than acoustic in nature.

Cavities, also known as cavity resonators, have reasonable lengths at the frequencies above about 150 MHz. Below this frequency, the cavity can be made to work, but it will be long and unwieldy. Like the crystals, cavities resonate at the fundamental frequency f_o, and at harmonic frequencies also.