How are diamonds more reliable than gold (and bitcoins)?

You should try to buy some diamonds, and then sell them the next week. You wont be getting anywhere near your money back. Unless of course you bought a rare diamond, which has been "cut" by a famous diamond polisher.

Diamond prices are held artificially high, by suppressing the production.

Bitcoins and gold on the other hand have a value, with which you can both buy and sell them. (minus a few % comission of course)