Here's why:

* Structure: Quartz (SiO₂) is a network covalent solid. This means that silicon and oxygen atoms are linked together in a continuous three-dimensional network by strong covalent bonds.

* Bonding: The silicon and oxygen atoms share electrons to form these covalent bonds. This sharing of electrons leads to a very strong and stable structure.

* Ionic vs. Covalent: In ionic compounds, electrons are transferred from one atom to another, resulting in the formation of ions. In covalent compounds, electrons are shared between atoms.

Key takeaway: Quartz is a covalent compound due to the sharing of electrons between silicon and oxygen atoms.