A pic would be helpful. But, if it's one of the older 'Economy' stones from Norton, it's likely aluminum oxide. The 'coarse' side will be a darker grey, and the 'fine' side a lighter grey.
The more recent SiC version of the economy stone is quite darker, almost black, as compared to the greyer AlOx version. Norton didn't switch from AlOx to SiC in that stone until 2008. So, if the stone you have is considerably older than 2008-vintage, and it is actually one of the 'Economy' stones, it'd very likely be AlOx.
Couple of pics here... In the first pic below, three of Norton's stones in their packaging - the one in the middle, labelled as '87933', is the AlOx version of the 'Economy' stone (the 'Fine' side shown is lighter grey than the 'Coarse' side, not seen). The larger stone at left is also AlOx. The one at right, labelled '87935', is a SiC stone, which is how the SiC version of the Economy stone would look (darker/black), as compared to the AlOx version.
In the pic below, some of my stones in their oil bath. The two larger ones at left are stones from ACE hardware. The greyer one at left is AlOx, and the darker/bluish-looking stone next to it is SiC. Also visible underneath the pink/white Arkansas stone is one of my 6" SiC stones, which is exactly what the Norton Economy stone would look like. An identical-looking stone was also sold at Sears for a while, under their own branding. I should note that the bluish cast seen in the SiC stones here is somewhat an effect of the camera's flash. In more typical lighting, the stone will look more black, although that blue tinge comes out somewhat in very bright light.
