What causes quartz to become more opaque or otherwise milky?

Quartz becomes more opaque or milky due to the presence of inclusions, such as tiny gas bubbles, liquid, or other minerals trapped during its formation. Additionally, the process of irradiation can alter its crystal structure, leading to a milky appearance. Chemical impurities or alterations during metamorphism can also contribute to this opaqueness. Overall, these factors affect the light transmission through the crystal, resulting in a more opaque or milky quality.