Lowest and least benefit

Meaning

The lowest and least benefit is not an admission that the material claims are worthless on their own terms. The passage calls them excellent, then compares them with the far greater knowledge of heavenly things. ‘As nothing’ expresses that relative hierarchy of value.

The English ‘lowest and least part of this mastery’ prevents schlechte from being read simply as defective or fraudulent. The things being ranked include improvement of metals and stones, not the knowledge praised earlier. The comparison is central to the work’s purpose: material success may be admired while still being subordinate, so the art’s value cannot be measured only in the production of precious substances.