Why is iceberg green in the line "As green as emerald" in "The Rime of the Ancient Mariner"?

Good question. One of our textbooks attributes this to an out of date and inaccurate source-book on the south pole.  The book claimed that icebergs were really green.  Of course, Coleridge, having never been to the south pole or seen an iceberg took this as truth and it found its way into the poem.

However, as I'm sure you noticed, as the poem progresses the imagery employed takes on a nightmarish or surreal quality.  So Coleridge may have also used the imagery of an emerald iceberg to help reinforce this imagery.