Apart from diamond being not rare as explained above (hence it is not precious stone), diamond now can be made in the lab.

Not only lab diamonds are much cheaper (China dominates this industry), they are actually more beautiful (no flaws).

Actually even before the introduction of lab diamonds, natural diamonds didn't have resell values much. They were sold at high prices because the prices were controlled (mostly by De Beers) by slowly releasing diamonds to the market. So if you own a diamond and want to sell it back to the shop, the shop won't buy it back high because they already have a lot of diamonds to sell them out.