Indeed, you are quite correct Edgar-Allans-Hoe!

In addition, I also read that once nanotechnology becomes more perfected, and widespread, diamonds (which are IDENTICAL to current diamonds dug up from Earth), will be extremely easy to precisely replicate.

This is because diamonds are simply carbon-lattice/crystal patterns. And upcoming technology will be able to make it so easily, that some say future windows in buildings and homes will all be made out of solid, large diamond crystals.

How valuable would a tiny diamond crystal on your ring be worth then? (No more valuable than a tiny sliver of glass is worth today.)

For more about nanotechnology, and mass-large-scale diamond production in the near future, I would recommend the books by the engineer/scientist Eric Drexler, if you want to know more.

In contrast, rare metals such as gold, silver, titanium, will continue to be difficult to create... in which you basically need a super-nova scale explosion to generate those metals (which are pure elements on the periodic table).

In order for gold/silver/platinum/titanium, to become truly as cheap as aluminium foil, would require humanity to begin mining asteroids in space (and would also require "getting lucky" and mining the right asteroid that just so happens to be rich in those rare metals).

Thus... in short... diamond will become extremely common place (since it's just carbon patterns), whereas rare-metals will tend to continue to hold their value most likely (assuming humans continue to value/appreciate rare metals).