Being fairly new to the hobby, this surprised me with slate. I expect it with wood since every tree harvested could be from a different location or age, even when the same species. Slate, however, I expected to be far more uniform. I’m no geologist, so it was easy to assume. And I made an arse of myself because of it. Bought a moderate size batch from a certain retailer and used it up. I was fairly pleased with the price/quality ratio so I ordered a second, larger, batch. Put a set in my fixture and started the job. Came back 20 minutes later and immediately cancelled the job. It was obvious by that point the previous setting were not going to work. It only took one piece to get it dialed back in but my assumption meant I lost three from the cancelled job. Live and learn.