They did seem like a strange addition to the story, didn't they? While critics of the film frequently cited the rock-like fallen angels from Arenovsky's Noah as a random change to a story that many people find sacred, these creatures follow a series of coherent and sometimes clever interpretation of Judeo-Christian tradition. However, their actual relevance to the story of Noah is pretty tangential, and even someone familiar with the actual cannon can be forgiven for not recognizing their origins.
First off, it's important to understand that introducing new elements to a story held sacred by so much of the world is not as unprecedented as it may sound. The story of Noah's Ark is only [46?—CHECK] verses long, so I don't know how anyone could expect a three hour long film to avoid focusing on one possible set of interpretations. Furthermore, the creation of supplementary stories to draw conclusions from and fill gaps in the Bible has always been a part of Abrahamic religion. The Jewish Talmud and Midrash—very important parts of the religion—are examples of this, as are countless Christian texts. Many classic elements of the story, such as the animals poking their heads out of the ark, are actually quite modern additions.
The origins of the rock people in Noah [2014 AD] come from one of those 46 lines. Genesis 6:4 says:
"The [Nephilim] were on the earth in those days, and also afterward, when the sons of God came in to the daughters of man and they bore children to them. These were the mighty men who were of old, the men of renown."
The Nephalim, as you may be able to gather contextually, are the basis for the rock people in Noah [2014 AD]. What does Nephalim mean? That's a tougher question.
The word is usually translated as either giants or fallen. but the word itself only appears one other time in the Bible (Numbers 13:33). It isn't a part of any post-Biblical version of the Hebrew language and it doesn't appear in secular documents. Giants is more common traditionally, but fallen makes a little bit more sense etymologically and also has a historical basis. But the identity of these strange creatures who “inhabited the world in those days” is left to interpretation.
The only clue is the cryptic description of “sons of God” and “daughters of men”. There have been fringe biblical commentators who have argued in favor of the Nephalim being the literal descendants of God, but this obviously hasn't had much traction in modern religion. After all, Christian theology is fairly dependent on what a big deal God's son being on Earth centuries later was. And modern Judaism pretty definitively rejects the idea of God having children. More take sons of God as a poetic reference to angels, which is supported by a similar reference to children of God in Job. The book of Enoch, the weirdest book you've never heard of, is in part focused around the Nephalim as the descendents of fallen angels, which it calls watchers.
The movie combines the two interpretations of Nephalim—the rock people are both giants and fallen angels. In a flashback the angels are shown to be made of light, but when they fell to Earth for helping humans create tools they literally changed into earth (rock or dirt). This is a clever way to ground the narrative's themes in its images (pun intended).
So do the rock people characters actually relate to thousands of years of biblical interpretation? Not quite. Noah  takes the idea of giant fallen angels and uses them to establish its impersonal interpretation of God, to build the ark and provide the completely necessary summer blockbuster battle scene. The one thing that the movie forgets to work them into is their actual biblical reference.
Attentive readers may notice the Nephilim only come up in Noah [600 BC-ish] in terms of propagating a race of mythical heroes. There is nothing to suggest that the giant fallen angels of Noah[2014 AD] can reproduce with humans, in fact it's strongly implied that they had kept away from humans since their fall. Noah  synthesizes a tradition of Biblical analysis brilliantly, but forgets to connect to the actual Bible.