Why the diamond is a good monetary asset ???

Because the diamonds, and especially the colored diamonds, are not UNDER CONTROL .....

Fiat Currencies, Supply of Physical Gold and Crypto-Currencies are all under control .... Debts is a major element of the current financial system !!!

To consider colored diamonds as a good monetary asset, it s important to remember it s a "free" asset. 

Colored diamonds have not even a Rapaport index ....