It's a precipitate rock, it's formed either the dissolution of limestone or low on my list is via marine organism deposition (kind of like Flint), so it's primarily silica. I can see some banding in the swirls, that's similar to growth rings on a tree or a stalagmite/stalagtite, it layers over time, the quartz which is Si02 as well probably after effect on the deposition (where it was sitting). It's a great specimen. Take it to the local U with a Geo Dept and see if they can slice it for you. Or if you know someone with a water jet or water jet tile cutter to see if they can slice it. It's probably awesome in cross section and would make fantastic drink coasters.
