Why is ordinary ice white, but pure glacial ice blue?

Question

Just read this question in a Carl Sagan book. I'm curious now.

Answer 1

air bubbles, primarily.  the bubbles scatter light and give it a white coloration.  In glacier ice, recrystallation and compaction have squeezed out the air, so you see the true blue color of the water (water is a naturally blue substance, which seems obvious but many people do not know this).