What is a peculiarity of ice?

If you freeze boiled water, the result will be a colorless, clear cube.

Conversely, freezing ordinary tap water results in an opaque, white colored ice cube.

The opacity is due to salts and gases dissolved in the water. These two factors prevent the formation of a perfect crystal lattice and a completely transparent ice cube. As the water freezes, these impurities concentrate inside the cube, forming defects and making it whitish.

Conversely, distilled boiled water (free of impurities) expels most of the gases dissolved in it (since the solubility of gases decreases with increasing temperature). Once frozen, the resulting cube will be nigh transparent.