Complex sentence: The most common crystal used was galena (PbS, lead sulfide), a naturally occurring ore of lead and a semiconductor with a small bandgap of about 0.4 eV, used without treatment directly as it is mined.
Split into simpler sentences: The most common crystal used was galena (PbS, lead sulfide), a naturally occurring ore of lead. Galena is a semiconductor with a small bandgap of about 0.4 eV, and is used without treatment directly as it is mined.