From A Lump Of Clay...

Another type of matter replication transforms a substance known as protomatter into the desired object. In many respects, protomatter resembles the basic shapechanging nanocolony known as UFog (see above) because it can be transformed into almost any object. Protomatter is a generic base from which all other matter is replicated; it can be transformed and molded as the replicator’s computer system sees fit with no need to transform one element into another. The use of protomatter is one of the most efficient forms of matter replication and has the beneficial side effect of being environmentally safe.