diamantine

Consisting of or resembling diamond.

This was the worke of God, that hée might ſeparate the Pure from the Impure: that is to ſay, that he might reduce the more pure and Ethereal Mercury, the more pure and inextinguible Sulphur, the more pure, and more fixed ſalte, into ſhyning and inextinguible Starres and Lights, into a Chriſtalline and Dyamantine ſubſtance, or moſt ſimple Bodie, which is called Heauen, the higheſt, and fourth formall Element, and that from the ſame, the Formes as it were ſéedes, might be powred forth into the moſt groſſe elements, to the generation of all things.

[…]the Mine-men having little Iron-hooks, with which they fetch out the Diamantin-oar:[…].

Hard as diamond, adamantine.