Re: What substance can cause chemical weathering of diamonds?

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.)