Not every diamond can be considered a stone for investment, because not every stone has the right requirements to be in this category. There are three sectors for diamonds.
Diamonds for industrial use
(80% of the market) = rough and very poor diamonds used for specific productive process.
Diamonds for commercial use
(20% of the market) = diamonds of quite good quality used exclusively for the jewelry market.
Diamonds for investment
(1,5% of the commercial market) = Diamonds of excellent quality, in this category there are only the best stones that will increase their value over time.
The rarity of the diamonds for investment