KPK, Ouch! Unless the customer was being blatantly reckless with the
piece and had been warned of the fragility of the product prior to
having it handed to them, the store should accept responsibility.
This advise has no legal backing but come on we’re talking customer
service here. Besides, where was the counter pad?
If the customer steps up and offers to pay for the stone use it as
an opportunity and offer to split the price. This will make the
store look good. Depending on the size and price of the item you
have also covered your cost. Give them the broken item. They paid
for it. Now offer to recut it in to a usable stone if possible and
mount it into a new piece at full retail.