Pennsylvania Blue Stone is only here in PA... hence the name. Because of this exclusivity there have been high demands for this stone elsewhere in the U.S. and it has been sold at quite the premium. The situation created a problem for us here in PA because all the nice stock was being shipped across the country and the junk was left with us causing less attractive and less reliable finished product. The pieces came off square, off size and de-laminated. Ultimately over time the material became less desired which gave way to a more manufactured Blue Stone. It's called Therma-flare and is cut totally then sent through a torching process leaving a totally perfect piece rather than the natural guillotine process. The difference is the Therma-flare looks more like a manufactured tile rather than a natural stone...I think maybe at that point you save yourself thousands and just get some tile from the Depot. If I order stone then that's what I want it to look like.
The good news is that Natural Cleft has been more available recently if you know where to look...I happen to know a few places!
A few things to look out for if you're getting some Flagstone installed:
A. Make sure your mason is not using lime in the mix, there should never be lime in a flat application. (a lay down install)
B, Every piece should be set twice. Your mason should set and level the piece and then lift it to ensure there are no voids in the mortar bed, then glue the back with a cement glue. (portland cement with no sand)
C. The joints should be tuck pointed with a pointing iron, not pastry bag or piped in.
D. If the application is on existing concrete then a bonding agent should be used.
If you have any other question contact E.L.D. Masonry, I'll be happy to talk with you about it.