The first three Pokemon games had a rather simple pattern of naming them after colors.

So on the original Gameboy, we had:

1.-2. Red - Blue/Green (depending on the region)

3. Yellow

After that, Nintendo evidently decided that was a little bland, and they decided to name the games after well known precious materials, since the sales of the games had, well, shown the series itself was a well known precious material (just guessing, though).

So on the Gameboy Color, we had:

4.-5. Gold - Silver

6. Crystal

Then when it came to the GBA, Nintendo realized they wanted to stick with the pattern, and even top the materials, to show the series was advancing (although some would say that was one of the only ways the series was advancing)

7.-8. Ruby - Sapphire

9. Emerald

Now with the DS, it seems to be reaching a head.

10.-11. Diamond - Pearl

12. Platinum (which has just been announced)

What the hell? If they stick with this pattern, what's left? It would have to be well known, and even more precious than those three.

13.-14. Uranium - Plutonium?

15. Um, what's more precious than those, that most people have actually heard of?

Of course they can break that pattern, since there isn't much left. Then again, it would also be good to have a major change in the series as well, but that's a whole other discussion.