I'll let a couple of the meer experts get into more specifically. The bulk of the pipes on the market, especially unsmoked, are Turkish meers. Their blocks have 6 different quality levels, largely centered on purity, color & porousness. The problem is that no one sells the pipes and tells you specifically with carver used which grade on which pipe. Certain carvers like Baki have a reputation for using the best stuff, so I assume it's Grade 5 or 6 but I don't know. I'm sure price is probably the best indication of quality but obviously, it's not perfect.
African production has closed down in recent years but there are numerous ones still out there in the estate market. Generally, they are more durable but less porous than Turkish meers. I think I've heard that the best African meers are close to Grade 3 or 4 Turkish meer.
As a specific to Roth's meers. The old Peterson meers which came from the Isle of Man were African meers and when production shut down, the newer stuff is now Turkish meers. The prices of the African Peterson is pretty reasonable and comparable to estate system pipes. The new Peterson Meers are about $300 USD and about the price of a Baki.
