Utility and Marginalism

In order to account for the high market price of a relatively useless, i. e. in comparison with water, item, such as diamonds, some advocates of the Use Theory of Value, 'Marginalists', have developed the category of 'Marginal Utility' to accommodate non-vital usefulness, thereby, according to some of them, reinforcing a Capitalist response to Marxism, insofar as the latter relies on the Labor Theory of Value.  However, typically lacking in Marginalism is a justification for the stretching of the concept of Utility to include the objects of mere wishes with those of real need.  That absence is perhaps fatal to the doctrine, since it leaves the term 'Marginal Utility' question-beggingly equivocal, and, hence, a ripe target for the Marxist who interprets the high market price of diamonds as a symptom of Capitalist mystification.