Final answer:
The mystery element is a nonmetal because it is a poor conductor of electricity and is nonlustrous.
Step-by-step explanation:
The mystery element that is described as a nonlustrous solid and a poor conductor of electricity is a nonmetal. Nonmetals are elements that cannot conduct electricity or heat well and are typically dull in appearance. Examples of nonmetals include sulfur (S), phosphorus (P), nitrogen (N), and oxygen (O).