Opal gets its color from the micros-structure of the material. Small
spheres of silica, usually less than the width of the wavelength of
light, interfere with the incoming photons and disrupt them… This
changes the wavelength of the light and you we see this as color
change. These spheres are packed with great regularity, like ping pong
balls on a tea tray.
There’s usually some water included in the material and some opals
are hygroscopic (water hungry. I’ve seen opals that will stick to your
If these opals get into contact with water that’s got small particles
in it they will absorb water and dirt.
Most bacteria are far too large to enter the pores of the material.
Hydrogen peroxide, however, will enter the opal and then break down
into oxygen and water. The highly reactive oxygen will bind with
anything that’s around to form a white oxide (usually white). This
accounts for the milkiness.
I’d suggest that your opals are probably already contaminated with
bacteria (ever tried going to the loo down a opal mine?) and that a
good wash in clean water with a little mild antibacterial cleaner
(mouthwash) for no longer than a few minutes will clean the surface
and allow you to feel safe about cutting.