Note: "the quality of objects made by the spell is commensurate with the quality of the raw materials." Is sawdust high quality wood? No. That says to me that if you use sawdust, you're goign to end up... with the equivalent of modern compressed fiberboard, like used in lots of IKEA furniture.I would say if you could use Fabricate to make a ball of wood from sawdust, you could make a marble of ruby from ruby dust.
Do that with ruby dust.. you get this cloudy ball of compressed bits, not a single flawless crystal.