Yeah I've read all of that. There are very limited resources that I have found on the matter. Every cutter/supplier also seems to have there own sizing and grading scale. Prices seem to be higher based on availability "due to aging process". You can get good to great quality from different countries. Plateau is better "fire" grain. For rusticated pipes that doesn't matter. Hence the question about preferred briars, if known (I know it's not always disclosed by makers).I think it's more the grading than where it comes from. Some info here:
Mark Tinsky - Briar Blocks for Pipe Kits - American Smoking Pipe Co.