I grew up in a little town called High Falls which is about half way between Kingston and New Paltz in the Mid-Hudson Valley of NY. The town is named for a waterfall, which is now the site of a Central Hudson hydroelectric plant, but has a long history of industrial use. Down by the falls is a decorative millstone laying on the ground.
The millstone is made of a local rock called the Shawangunk Conglomerate - it's a hard, white rock that forms a ridge extending from near High Falls southwestward between the Rondout and Wallkill Valleys past Port Jervis and into the Delaware Watergap area of PA and NJ. Nearby, in the village of High Falls proper (across from the church), is another millstone with a slightly different pattern.
Perhaps they were used to grind different types of grain, I don't know. What amazes me about these millstones is that they were carved by hand! The Shawangunk Conglomerate is composed entirely of quartz - quartz pebbles and sand cemented by quartz (silica) cement. It's harder than granite.
Wandering the Shawangunk Ridge (in this case on Mohonk Preserve property), it's not uncommon to stumble upon old millstone quarries like the one below.
Millstones were likely mined here by the local homesteader because of suitable fractures in the rock. Slabs of rock would be taken out and then shaped into a rough disk. Why was the one here left unfinished? Who knows. Maybe the chip in the lower-right ruined it. Imagine working for months on this and then having it crack the wrong way!
Here's a nicely finished disk (you can't see it, but there's a hairline crack in it making this one useless as well). This one's at the Mohonk Preserve Visitor's Center on Route 44/55.
One of these days, I plan to write something up more formally about the old millstone industry here. There's a lot I don't know and I think it would be interesting to research a bit.