Rationality and Ostentation

In the standard study of the phenomenon, the purchase of diamonds at exorbitant prices is typically accepted as 'rational' behavior.  However, since typically absent from these studies is a definition of 'rational', that classification is difficult to assess.  In contrast, that behavior does not measure up to two of the most prominent concepts of Rationality.  Spinoza would not accept it, since he defines Rational conduct as the deliberate promotion of one's Health, which does not seem to include the purchase of diamonds in order to show off.  Kant would not accept it, since he defines Autonomous Respect as the defining characteristic of Rational interpersonal relations, whereas ostentation for any purpose is Heteronymous behavior.  So, at minimum, the classification of purchasing expensive diamonds to achieve some social status as 'rational' is vulnerable to challenge.