Complex sentence: Its most stable form, the grey trigonal allotrope, is sometimes called'metallic' selenium because its electrical conductivity is several orders of magnitude greater than that of the red monoclinic form.
Split into simpler sentences: Its most stable form, the grey trigonal allotrope, is sometimes called'metallic' selenium. This is because its electrical conductivity is several orders of magnitude greater than that of the red monoclinic form.