Old icebergs maybe hundred and thousand of years old. Many years of falling snow consisting of snow crystals by the countless billions act like a tiny mirrors and reflect the light. Some icebergs formed by freezing ocean water instead of snow areas are full of tiny bubbles.
Beautiful bluish streaks that appear in some icebergs are caused by the refreezing of melt water that previously filled very old glacier ice crevasses. That very dense very old captures the sun light allows only the high energy blue wave lengths to escape. the beautiful blue icebergs change color and intensity according to the position of the sun.