In reality, whether something is expensive or not is determined by market forces. Whether or not diamonds would have been inexpensive are not a part of the present reality because de Beers used to control a large majority of the supply of diamonds. If anyone wanted to sell diamonds, they had to play by de Beers's rules, whether they thought they were fair or not.
In yesteryear, emeralds, rubies, and opals used to be the more sought after gems, however a tightening of the supply of diamonds by de Beers in addition to a marketing blitz started in the 1950s that started to establish the idea that a diamond was the only acceptable wedding gemstone.