I'm not a geologist, but I've learned a lot of obsure geologic stuff in using "pretty" rocks on table tops.
The table top above features a polished slap of shinny black rock for the focus stone. The other stone is Lyons sandstone from quarries in Lyons Colorado, not far from my home in Boulder. The rocks are surrounded by a used whiskey barrel hoop. The top is supported on three rod steel hair pin legs.
When I first started showing this table at local art fairs, I put on the label that the rock was pollished granite. It turned out that there were a lot of geologists at that event and many of them told me that, no the rock was not granite; it is gabbro. To which I replied, what is the difference between gabbro and granite? The answers quickly got into geologic details like mineralogy, grain size, and other stuff that didn't really satisfy my curiousity.
After talking with geologists and googling a bit I leaned that both granite and gabbro are formed when upwelling magnma cools. Granite is formed mostly under continents; gabbro is formed under oceans.
This table has been sold. I don't have any more slabs of black gabbro or granite, but I do have a spectatular slab of pollished hematie from a mine in Minessota (below). If you'd like me to include this slab in a table top for you, get in touch: Bill@WKPaccentTables.com 303-956-3576