The demand in countries like India and China is rising. Diamonds and jewelry is the modern dowry. http://en.wikipedia.org/wiki/Dowry
Its inherent value is that it shows people that the family is well to do.
Things may change with the advent of artificial factory made diamonds (already showing up, but not in big quantities). But it won't ruin the diamond prices, just create two diamond categories. It should follow the trends that the pearl industry followed after the advent of cultural pearls.
Source: my Dad is a diamond trader.