Consumers are rational in the purchase of diamonds - their purchases are based on buying the diamond that shows the largest amount of wealth for the given stone

Consumers are rational in the purchase of diamonds - their purchases are based on buying the diamond that shows the largest amount of wealth for the given stone. Irrational would be paying more money for a smaller diamond. Responding to social norms creating by a marketing campaign is many things - strange, definitely - but it's not irrational.