Sure, at some level it probably is a "natural fissure". And it's also definitely an "unacceptable crack". Those 2 terms are not mutually exclusive. Hope you haven't paid yet....or paid with a credit card.

Thank you for your comments and in helping me feel justified in my anger about this. I stopped payment on the final portion of what I owed this man after he told me I would only have one slab at his shop to choose the replacement from. Unfortunately that was only one third of my payment to him. I shoulder some of the blame because I had asked to see the slab before he cut it and I didn't follow up. I wasn't fully cognizant of the issues that slate might have (it also has big black marks I wasn't aware would be there - see photo). His attitude and demeanor however, have been unacceptable and almost shocking given the nastiness of his responses.

keep on him. you are justified. slate is very, very fragile and prone to cracks during transport and install. I'm sure his guys were responsible for this, hence, you shouldn't have to pay for it. Sometimes business owners like this try to bully their way over nice people like you. tell him to get his butt out to your house and replace the stone or you'll be taking him to small claims court and filing a complaint against his license. that ought to wake him up a bit.

It's a crack. Oh the joys of remodeling.

It's a crack, he needs to replace it.