Sixty percent would be low grade, thirty would be of medium grade, and the final ten would be high. Or the formula could be modified to make it easier/harder to get the best type. And variations in each resource could make high grades more valuable. Even different formulas for different regions.
That would also mean that the number of all resources would now be tripled. Instead of just wood, you could have poor, fair, and good wood. And each type would remain separate in your inventory. The types would also be displayed when examining objects made with those grades.
Which brings me to the main point of having different resource grades. Tools, objects, buildings, etc. made with different grades would have according attributes. Low grade shovels would be able to dig less than medium or high. Low grade buildings would hold less weight. But other things could relate to resource grades.
If skills were to be introduced, being better at gathering would allow you to get more high grade resources and less low grade. Better tools would increase your odds even further, but by a smaller amount than the skill.
If erosion were to be implemented, high grade tools, etc. would wear down slower than low grade tools.
End Result: Quillanoi stone could actually be the best in the world. Well, assuming everything stacks up right.

