Here's a small tidbits in economics - the scarcer the good, the higher the price.
So technically emeralds should be higher in price than diamonds due to scarcity.
Enter De Beers and "a diamond is forever" campaign, which basically created the modern engagement ring in the process.
So basically, via marketing not economics, diamonds skyrocketed in value and price. Well, not anymore.
It seems diamond prices are in freefall, due to many reasons. The first is that lab grown diamonds are entering the market in full force. Considering a lot of diamonds have politically questionable origins, well, when you know your diamond is coming from a lab all ethical reservations seem to fall. Also, it seems millennials are diverting their spending more towards events and travel as opposed to jewelry and expensive objects.
Sure, no one is talking that diamond trading giants are going bankrupt, but apparently prices are in such freefall that India, the place where 90% of diamonds are cut and polished is giving heavy signals for mines to decrease their productions. Also Alrosa and De Beers are both taking precautionary measures - the former stopping all sales, the latter allowing customers to refuse stones they were technically bound to buy.
Sure, if you take the Alrosa logic, you play with supply, ergo the demand will increase along with the price (mind you this brings us back to the beginning and to the fallacy of the price in itself had it not been for the De Beers marketing campaign).
But hey, they'll always be a girl's best friend. Or a Shirley Bassey enduring gem.