Photos would help. To be clear, you viewed the slab and commented on the crack at the time? And you purchased the slab that you noticed had a fissure in it?

Thank you for your response. I have attached two photos although it is very hard to see the crack in the pics. I noticed what looked like little cracks within a day or two after installation. I was reassured that what I was seeing is normal, no problem etc. I now have one long crack which is upsetting and unsightly and I am concerned that the other "little cracks" I see will also spread and I will be left with this awful slab of damaged island.
I certainly don't recall any cracks in the slab I chose at the lot.

I had no problem seeing it and it looks like a crack to me!

Certainly tough to tell from the pics whether a crack or fissure, at least for me.

Eighteen inches for a crack in granite is disturbing. I would contact the fabricator and let them know what is going on, and let them offer suggestions - knowing in your heart that you want it replaced. You do not want anymore time to pass, it's been 3 months.

You have two things to consider. First, if it's a crack and you don't want it filled, you'll need to fight to get it replaced. Often cracks can happen along fissures, but the fabricator should have brought that to your attention, or at least addressed your concerns more seriously at the time that you brought it up.

Second issue, if you want the granite replaced, you may be hard pressed to find another piece of granite from the same lot. Just cause the granite has the same name doesn't mean it will look the same. Granite slabs are usually cut in 10 slabs per lot. Since it's been 3 months, you may not be a able to find another slab from your lot. Each 10 slabs have lot numbers and then cut numbers. So you may have lot number 1775, cut 1;2;3 etc. Since your granite has a lot of movement in it, cut 1 can differ greatly from cut 10, even in the same lot number.

I suspect your fabricator will suggest filling the crack with an epoxy.

Tough decision, good luck!