In this paragraph, the rock represents the human mind without any external influences set in place. It represents the mind not yet afflicted by hypnopaedia and the conventions of the World State.
In the society established by Huxley, the entire world population is under the control of the directors of the hatcheries and the world controllers. As soon as the children are born, they are conditioned into instinctively accepting the values of the society. "...drops of liquid sealing-wax, drops that adhere, incrust, incorporate themselves with what they fall on, till finally the rock is all one scarlet blob." The drops of wax represent the myriad of actions taken in order to conceal the true values inherent in each individual. however, despite the omnipresent pressure to stick to conformity, there are some exceptions in which the rock nearly breaks through the wax.
Helmholtz Watson is a perfect example of an "Alpha plus" of the society. However, his quality that causes him to "break through the wax" is that he is too smart. From our introduction to Helmholtz until the end of the book, he is depicted as a very deep thinker and a romantic. He frequently complains about how he has an unexplainable feeling of which he is not satisfied by his role in society and that through his rhymes, he wants to somehow write something meaningful. As a result of his constant search for his true emotions, at the end of the book, Helmholtz writes a rhyme on loneliness, which signifies that he has finally broken through the wax and revealed his "rock."
Similarly, Helmholtz's friend, Bernard Marx, is another character portrayed who breaks through the wax or has a thin wax layer. Throughout Brave New World, Bernard is always noted for his incongruous nature, both physically and psychologically. It is his small size that causes him to think differently from the conditioned majority. Examples of Bernard's inconstancy can be seen in the scenes with Lenina Crowne in which he talks about the values of true love and monogamy. In these portions, his "rock" is exposed to the readers, but we are still lead to believe that a wax layer continues to exist over his "rock." For example, when he becomes the guardian of John Savage, he goes back to enjoying the conventions of the society and turns his back on his previous beliefs. So, though Bernard has a wax layer, it is thin enough so that the rock beneath can be perceived and expressed.