The other thing about pokemon titles is that they emphasized parts of the game.
gold and silver (gen 2) introduced the Steel type
ruby emerald and sapphire introduced drastic improvements to the series with like 80 new moves and some hundred new pokemon.
gen 4's pearl diamond platinum, one of the main things introduced was the pokeradar which enhanced shiny pokemon hunting.
So, while they all do seem to be more valuable than the next, they also have specific reasons for their names.
Because there is just about nothing rarer than the last three titles, except for radioactive material, I would think they would go something to do with moon earth and sky like xxain wrote above.
However, those three mentioned in the OP are all shiny, all have various colors (ie topaz can be almost any color) and are pastel colors in their normal form.
This could mean that shinies will be back, and perhaps with new variations (multiple shiny types), and will probably introduce new types.
