Final answer:
The substance is a covalent network solid, known for its very hard structure, high melting point, and non-conductive nature due to the network of covalent bonds that hold the atoms in a continuous, organized structure.
Step-by-step explanation:
The substance described is very hard, has a high melting point, and does not conduct electricity, even in the molten state. These properties suggest that the substance does not have free electrons or ions to conduct electricity, which is a characteristic of covalent network solids. Covalent network solids are formed by a network of covalent bonds that hold the atoms together in a continuous structure. They are known for being extremely hard, having very high melting points, and being non-conductors of electricity in both solid and molten states. Examples include diamonds (a form of carbon) and silicon carbide. These properties of solids are due to the types of particles they consist of, the arrangements of the particles, and the strengths of the attraction between them.