Here's why:

* Diamond: Diamond is a giant covalent network solid with strong covalent bonds between carbon atoms in a tetrahedral structure. These bonds are extremely strong, requiring a lot of energy to break, resulting in a very high melting point of around 3550°C.

* Silicon dioxide: SiO₂ also has a giant covalent network structure, but the bonds between silicon and oxygen are slightly weaker than the carbon-carbon bonds in diamond. However, SiO₂ has a more complex structure with alternating silicon and oxygen atoms, leading to a more tightly packed arrangement and stronger overall interatomic forces.

This stronger overall network structure in SiO₂ results in a higher melting point of around 1713°C compared to diamond's 3550°C.

Therefore, the melting point of silicon dioxide (SiO₂) is lower than diamond.