Ya know, stone breaks and is repaired at installation all the time.

But...usually the fix is invisible. If you feel kindly, you can let them try to fix that fix, but I'm not too optimistic...

Yikes. You are right to be questioning this and it's completely unacceptable work. And for the installer trying to pull a fast and not tell you? Not right! Go back to the supplier and installer and ask them to make this right. Hopefully they will. Then rate their service everywhere on the Internet. With names

Update: I sent the installer an email last night (with photos). He responded and is basically denying that there is a crack, claiming that it's a "natural fissure". He also put it on me that I didn't want the stone oiled (I picked the stone because it's beautiful unoiled). He's obviously not owning up to anything. I shouldn't have to oil my stone to cover up his mistakes. Clearly you don't always get what you pay for!!

In some states, putting a "stop payment" on a check give for work done is against the law. You need to be sure you're not on shaky ground here.

No, he's not going to own-up to his mistakes. This is why it is SO important to be on-site when installations such as this are done.

"Oiled"? What is that material? Soapstone?

And no way on those cracks. Hell no. I'd pay for nothing less than a proper replacement. They screwed up. And how deceitful to not own up to it when it happened.

Oiled stone? That makes no sense. Soapstone is generally the only stone to ever be oiled.

A good quality fix of a fissure or a crack is undetectable. It will be stronger than the surrounding stone. Quarries repair stones all the time and send them along to slab yards and no one is the wiser. A well repaired crack is acceptable, and even to be expected in some stones. Some stones are just prone to cracking.

Thats not a well repaired crack. They should strive to do better.

But, because you stopped payment, they might not. You set yourself up as an adversary first thing, rather than a reasonable customer who just wants things corrected. Stopping payment was very uncool. If you’ve vetted your fabricator on the front end to be reputable, you just spit in his face, and virtually called him a crook. That's totally not the right way to handle that.

The installer is also the distributor? I find that odd.

What is the limit amount for Small Claims Court in your state? That might be an option for you.

Those cracks are installer/fabricator error. Everyone has to pay for their mistakes.