Highland Granite is ONLY in wool and is made by Lochcarron

Granite City, Silver Granite, and Grey Granite (all the same pattern... slight variations in weaving process and/or material... PV vs. Wool) are woven by Marton Mills.

Admitedly, MOST of the grey and black tartans look similar. There's no color to make any 1 THAT much different from another. It's sort of like the "Shadow Tartans"... no matter what mill you get them from, they all look pretty much the same since it's black on black (or navy/navy or cream/cream or white/white...).