MINING

The blog Priceonomics takes diamonds out to the woodshed for being no-good, darned-awful investments.
The mark up at the store is 100% to 200%, scarcity is artificially created by cartels, and unlike gold or silver, diamonds can't be easily traded:

A diamond is a depreciating asset masquerading as an investment.
As with televisions and mattresses, the diamond classification scheme is extremely complicated. Diamonds are not fungible and can’t be easily exchanged with each other. Diamond professionals use the 4 C’s when classifying and pricing diamonds: carats, color, cut, and clarity. Due to the complexity of these 4 dimensions, it’s hard to make apples to apples comparisons between diamonds.
But even when looking at the value of one stone, professionals seem like they’re just making up diamond prices.
So let’s be very clear, a diamond is not an investment. You might want one because it looks pretty or its status symbol to have a “massive rock”, but not because it will store value or appreciate in value.