Final answer:
Quartz's hardness and high melting point are due to its structure of interconnected SiO4 tetrahedra, making it very stable and strong. Quartz melts at around 1600 °C, forming fused silica when cooled rapidly.
Step-by-step explanation:
The best explanation for quartz's hardness and high melting point lies in its chemical structure. Quartz is a mineral composed of silicon dioxide, or SiO2, which forms a rigid framework of SiO4 tetrahedra. These tetrahedra are linked together in a continuous network, creating a very stable and strong crystalline structure. This structure is what makes quartz very hard and gives it a high melting point of around 1600 °C.
In nature, quartz can be found in various polymorphic forms, and trace amounts of elements like iron can give different types of quartz, such as amethyst, their distinctive colors. Additionally, when quartz melts and cools rapidly, it forms an amorphous solid known as fused silica or silica glass, which has useful properties such as low thermal expansion and high transparency to ultraviolet light.