I know there are a few similar posts, but can't find anything that answers it directly.
I'm looking for somewhere in NYC that will mount my diamond, and provide a warranty against damage during the mounting process. It would also be useful to get an idea of cost.
It will cost you anywhere from $1-$3 per point in NYC. Meaning a 1ct diamond will run you from $100-$300. You may find some cheaper, or you may find some more expensive. My advice would be to search for a good bench in NYC, there ar emany pscope memebers who can direct you to one of those.
The problem with your question is that unless you bought the stone from the place you're having it set, no on is going to take any sort of liability if something should happen to your stone. You need to get the stone insured before you have it set (again, unless it's being set by the vendor you bought it from), otherwise you take a bit of a gamble that nothing will happen to it. Godd luck -
Jewelers Mutual will sell you insurance that covers the possibility of breakage during setting. As far as I know, they are the only one of the insurance companies that has a product that includes this. Most jewelers do not warranty things they didn't sell but it may just be a matter of negotiating the fee.
I'm going to guess that this is a new purchase from some NYC based dealer. If so, talk to the seller about having it set. Most are pretty well connected with a network of setting suppliers and benchworkers and a lot of them WILL warranty work on things that they're selling.