Do the color of the thuringian stones determine the grid of the stone? I own three thuringian stones and they are different colors, one is a lighter greenish color and seems to be softer. The other two are dark grey and seem to be harder and higher grid. The darker ones seem to take a little longer to slurry and the light green is very easy to slurry. Any experience and educated information is appreciated, I'm still learning, thanks everbody.