Here's the scenario:
you decide to invest in a rare gem and you approach a registered diamond broker to do the sourcing for you. The firm duly comes back and says that they have found something within your price range- say $20k.
If you agree to invest with them and buy this stone, they will send you the diamond and all the formal paperwork proving that it is what it says it is and that the value is $20k as stated.
Alternatively, they will store it in a secure vault for you free of charge. If you use the broker for the resale, they will take a 5% cut of the final price.
You duly go ahead with the deal and purchase the diamond. You then hold onto this gem for 3 years and sell it privately at a price significantly higher than you originally bought it.
In all of this, what was 'in it' for the diamond broker? Where did they make their money???