Here's why:

* Silica Composition: Opal is made of silica (silicon dioxide, SiO2), just as you described.

* Milky White Appearance: Opal can come in a variety of colors, including milky white. This is due to the way light interacts with the silica spheres within the mineral.

* Other Characteristics: Opal can also have a play-of-color effect, where it displays a range of iridescent hues when light shines on it. However, milky white opal typically lacks this effect.

Let me know if you have more information about the mineral, and I can help you narrow down the possibilities further!