This stuff from Lee Valley says its aluminum oxide grit (although it looks blue), but seems to cut better & last longer than the 'brown crap' I have, which just seems to dull & lose its grit off the backing after very little sanding.
http://www.leevalley.com/en/wood/page.aspx?p=20182&cat=1,42500
btw - I'm talking loose stock off a roll here, not closed loop belting for sanding machines. I also checked the label on my 'brown crap, its KBC Economy roll. I think that explains the price difference between this & the good stuff they offer, but I have another 48 feet to use up. When I see the punishment those knifemaker sanding belts go through, maybe I should just buy some belts & cut it into pieces for handwork. Guessing the backing is probably more rigid & less conformable than the typical cloth, like old school emery paper. Do they even make emery paper anymore? I rarely see it.