|MadSci Network: Earth Sciences|
This isn't my area of expertise so you may wish to seek another opinion. However a little checking shows that the issue here is that bonds between atoms in the diamond crystal structure are extremely strong so at normal temperature and pressure natural solvents like carbonic acid won't break those bonds. You could over a long time abrade an exposed diamond away perhaps (physical weathering) but I don't think typical "natural" solutions will make much progress. If you really want to damage your diamond I suspect a good plan is to allow it to subduct into the earth's interior and heat up and melt. One other thing I'm aware of will degrade diamonds over time, but very slowly: Naturally occurring uranium and thorium decays radioactively and can under the right circumstances change a carbon-12 atom into a carbon- 14 isotope. This will radioactively decay itself in a few thousand years on average; so the earth's natural radioactivity is gradually degrading diamonds. (This process is sometimes overlooked, leading some people to erroneously conclude that diamonds with trace amounts of carbon-14 are only tens of thousands of years old, but in fact diamonds are generally tens or hundreds of millions of years old.)
Try the links in the MadSci Library for more information on Earth Sciences.