Iron sulfide is a chemical compound (different from a mixture of Iron and Sulfur, show by its different physical and chemical properties). It is made of iron and sulfide ions.
A dielectric material (dielectric for short) is an electrical insulator that can be polarized by an applied electric field. dielectric resonator oscillator (DRO) is an electronic component that exhibits resonance of the polarization response for a narrow range of frequencies, generally in the microwave band. It consists of a puck of ceramic that has a large dielectric constant and a low dissipation factor.
A lossy dielectric, on the other hand, is one that irretrievably absorbs some of that energy (and eventually converts it into heat). Thereâs no such thing as a perfectly lossless dielectric, but many materials (ceramics, glasses, some plastics) have very low loss and can be considered to be effectively lossless.