Diamond may not a good investment, unless it has a certificate. it really depends on 4 C.
Carat, Cut, Charater, Color.
If the Diamond is small which means the value will be small as well. Also it really depends on the market. Because diamond is only a necessary goods, it only a high value products.
It really depends the market needs and how big your diamond is. To double the price, it depends on the second hand market.