Diamonds are viewed as rare, valuable and considered a desirable object by many. The Tiffany advert shown links true love which can also considered to be rare, to diamonds. I think the advert also implies to men that for your one true love that a Tiffany engagement ring would symbolize your love, both in rarity and beauty.
Research has found that scarcity enhances the desirability of experiences and objects. Lynn (1989) primed subjects to think about expensiveness or the desirability of wine. Participants were assigned to scarce or available conditions, which were implemented by telling participants there was a large or small grape harvest for the wine. Participants rated their desirability for the wine on a nine-point likert scale on questions such as “How willing would you be to buy this wine for yourself?”. The study found that scarcity enhanced both the perceived expensiveness and desirability of a wine. Therefore, the use of the scarcity technique is implemented for both expensive and rare objects. However, ironically the more diamonds that are sold the less rare they are.
Lynn, M. (1989). Scarcity effects on desirability: Mediated by assumed expensiveness? Journal of Economic Psychology, 10(2), 257-274.