The basic conflict that exists between Ray and Mark embodies two different ways of looking at the world. Mark sees things in absolute and materialist terms. Being a businessman, Mark values practicality and profit. He wishes to take ownership of the farm so that he can turn a profit from it. More than this, Mark is rooted in the idea of "what is." Mark embraces the reality that is in front of him, depending on the verifiable and quantifiable, empirical data that exists in his own sight. Mark views what is and seeks to make it his.
Ray is cast in a diametrically opposite light. He is not excessively practical. Showing disdain for his previous job selling insurance and not very well versed in matters of money, Ray is seen as a dreamer. He embraces that which is not there and constructs reality as it should be, as opposed to strictly how it is. Ray is willing to take a chance on building the field, acting on voices, and pursuing a notion of the good that others might not be able to see. This conception of reality is essential to him, reflective of what he deems important, and animates his being. For Mark, the goal of consciousness is to appropriate that which is there and have more of it. For Ray, it is to uncover intangible truths that underscore our being and purpose in the world. Both are two different modes of reality.
The irony of "diamonds in your cornfield" is that Ray actually does discover something more valuable than diamonds. Naturally, as Salinger predicts, the farm makes money because "people will come." Yet, the "diamonds" of precious value for Ray is how the field is able to unite Ray with the spiritual basis of the world that he has lacked for so long. This operates in the sense of the general, in how Ray's dreams have been realized. Yet, it also operates in the realm of the particular in how Ray is able to find reunificaiton with his father. For Ray, the "diamonds" in the field exist in both tangible and intangible forms. It is irony because while Mark says it in a derisive manner, it is so very true for Ray.