Diamonds, crystals and Nature’s furnace

Meaning

Diamonds, precious stones and crystals are interpreted as glasses perfected in Nature's furnace. The furnace is a metaphor for formative natural heat, not a documented human apparatus used to manufacture gems.

I.8.3 uses these hard bodies to support an account of vitrifying virtue and the perfection of matter. It appeals to experience against a competing explanation, so the passage contributes to the book's argument about how hardness is produced. Its categorization of gems as glasses remains historical natural philosophy; it must not be represented as modern mineralogy. Nor does clarity guarantee freedom from every impurity: the later common-crystal experiment reports an unsuccessful attempt at purification, ending in vitrification with dregs.

The English retains the material names and the “ingenious Nature” personification. The connection to the later crystal failure is explanatory, not a silent correction of the earlier theory.