Read through the Geology thread written by karin_mt and take note of the tests you can do at the slab yard to try to determine what the slabs really are. Some stone yard workers just don't know enough to be answering questions! If you can take a sample of the slab home, that opens you up to doing even more tests at home, but most slab yards do not give out sample pieces.

I didn't get a sample piece of my Taj Mahal slabs before buying them at the slab yard, but once my fabricator had them I was able to get him to knock off a corner piece for me to test and use for backsplash research. I did all of Karin_mt's tests on my piece, so at least I knew what precautions I needed to take once the counters were installed. Mine turned out to be real Taj Mahal quartzite and pretty much "bulletproof".

Here's the articles that chispa is referring to.

https://usenaturalstone.org/definitive-guide-quartzite/

https://usenaturalstone.org/properties-of-quartzite/