Addendum

Some months ago, in my review of the movie Noah, I said I was aware of no direct antecedent to the film's portrayal of the fallen angels' transformation into stone after the fall. While I was going over some primary sources for an upcoming paper, I realized I had forgotten something.

The Pseudo-Clementine Homilies, a Christian work (possibly Jewish-Christian) originating before the 4th century CE, contains a version of the fallen-angels narrative in which the angels transform themselves into precious stones!

For of the spirits who inhabit the heaven, the angels who dwell in the lowest region, being grieved at the ingratitude of men to God, asked that they might come into the life of men, that, really becoming men, by more intercourse they might convict those who had acted ungratefully towards Him, and might subject every one to adequate punishment. When, therefore, their petition was granted, they metamorphosed themselves into every nature… So they became precious stones, and goodly pearl, and the most beauteous purple, and choice gold, and all matter that is held in most esteem. And they fell into the hands of some, and into the bosoms of others, and suffered themselves to be stolen by them. (Hom. 8:12–13, trans. ANF 8:272–274)

In this version of the story, the angels descend to test the righteousness of humanity, and their transformation is a voluntary one. The precious stones, gold, and purple fabric are clearly an allusion to 1 Enoch 8:1-2, wherein the Watchers teach humanity how to fashion dyes, jewelry, and cosmetics, which in turn lead to greater human wickedness, although the emphasis in the Homilies is more on greed than the temptation of female adornments implicit in 1 Enoch. In the Homilies the angels themselves are not corrupted until they later transform into human form, and take on the concomitant weaknesses and lusts.

I have no idea if Aronofsky might have had this story in mind when he imagined his stone Watchers. Most of his sources seem to have been Jewish, not Christian (and most can be found in Ginzburg's Legends of the Jews,) so I'd guess not, but it's such a strange and delightful variation on the Watchers tradition that I can't believe I didn't think to include it in my original review.