We have a lot of gravel in the midwest for concrete and roads. I remember when we visited New Orleans though, instead of gravel they used crushed sea shells. Seemed strange to me but I guess ya gotta use what ya got. I don’t know if they use the sea shells in concrete or not but wouldn’t surprise me.
Granite is indigenous to the area here.
Apparently seashells are indigenous to New Orleans. 
I also suspect that not all granite is monument or counter top grade, just like not all diamonds are gem grade.
Just an FYI:
I spent the better part of a week in a limestone quarry near Miami, FL. They call the limestone coming out as "New Limestone. The sea shells cleaved in such a way that they created sharp edges. When used in concrete paving - vary common in the area - those edges get exposed fairly easily and quickly.
As a result tires do not wear very well and it is known in the tire industry that southern FL is pretty rough on tires.
Truth is, there are many types of granite.
The best, most dense, most consistently grained granite is used for “surface blocks”. They’re polished to an extremely high level of flatness and used in coordinate measuring machines and the like as extremely stable and true reference surfaces. You’ll find them in every good machine shop, calibration shop, and even inspection shop.
Personally, I dislike granite kitchen countertops. Granite is hygroscopic, it absorbs fluids and stains. It needs to be treated periodically to avoid that.
Modern granite countertops are sealed, often right at the factory after polishing. The granite counter/vanity tops in my home are sealed and will easily last decades without needing anything beyond normal cleaning, no different than any other material used for those purposes.
We have a number of “optical” tables at work made from highly polished ~1ft thick granite slabs. They’re meant to be perfectly flat (for placement of optical elements used to direct light) and be resistant to any vibration that would disrupt the optical path. It took an entire team of movers with specialized equipment to move one of them a few months back when we needed to relocate it.