Glass is Amorphous SiO2. Glass has no anosotropy, can be used as substrates for optical applicaitons.

Quartz is crystalline SiO2. Quartz is very high optical clarity, especially in UV and IR ranges, can be used in optics and high-precision instruments. Quartz hs high melting point (~1700°C), excellent thermal stability — it can withstand rapid temperature changes.

Sapphire is a hard material known for its exceptional optical, mechanical, and thermal properties. It has exceptional hardness and durability. It iss econd only to diamond on the Mohs scale (hardness ~9). Sapphire is transparent from UV (150 nm) through visible to mid-infrared (~5 µm). Its melting point is ~2050°C. Sapphire has excellent thermal conductivity and resistance to thermal shock — suitable for high-temperature furnaces, plasma chambers, and reactive environments. In labs, they are commonly used for 1) optical windows, lenses, and domes for spectrometers or lasers; 2) Substrates for epitaxial growth (e.g., GaN LEDs).