Hi Phil - if you didn't purchase the diamond from them, then yes, it is very common for them not to take responsibility. While it's rare, diamonds do sometimes break while being set into jewellery. If you haven't purchased the diamond from them, it is completely at the jewellers discretion whether or not they are even willing to do the setting for you...what is the financial incentive to take that risk? In fact, you'll find that most won't even be willing to try and set it for you...too much risk for very little reward if they have to be responsible. I don't know what you're having set, or what they are charging for the service, but for the sake of argument, lets say that they're asking $100, and your diamond is worth $10k...if that stone breaks through no fault of their own (maybe your stone had a pre-existing durability issue that the jeweller was not aware of), no insurance company will pay out for that (assuming the jeweller even has insurance), and now the jeweller is expected to eat the cost of replacing that stone...
These are unfortunately the perils of buying loose stones without buying the setting from the same vendor - so for anyone reading this thread, you might want to keep this in mind - either purchase the stone and setting from the same place - or make sure that you have someone lined up that is willing to set the stone (again, likely without being liable) before you purchase a loose diamond, so that you're not stuck running around trying to find someone to set your diamond.
With your household & contents insurance, maybe speak to your insurer to see if they offer some sort of a short-term optional portable personal valuables cover? (most insurance only covers jewellery when it's at home)
Hope this helps