Hi Darrell

I have worked in the metal industry and I do know that there are at
least half a dozen different grinding wheel colors, and yes there is no
standard as to exact hardness and friability between different
manufacturers, but there is a general labeling standard on grit,
material and hardness etc. and wheel colors are also indicative on bond
strength/hardness, I have a nephew who makes his living in an R&D lab of
a large electronics co. as a fine machinist and grinding as his
specialty, and his take on the wheels he uses is that you really have to
know from which manufacturer the wheels are, and experience has taught
him they are not all the same, 2 wheels with the same color, grit size
etc. and from different manufacturers makes for 2 different grinding
characteristics.

I have a link here to some more info if anyone is interested

http://www.jjjtrain.com/vms/other_grind_wheels.html#8

Have fun and take care
Leo Van Der Loo

Darrell Feltmate wrote:
Chip
I believe that if you check you find that Aluminum oxide is a white
crystalline substance and the various colors that are for sale are added by
manufacturers for proprietary classification purposes.