Definition of "cornelian"

This proceedeth from the variety of air (commonly found in islands) which sometimes being moist, sullies and renders the cornelian pale or albescent, after the manner of glass, which when breath'd upon, is clouded with a pale whitish colour, but upon change to a drier air, which will often happen in a moment, the cornelian recovers its former brisk red colour.

1726, The British Apollo: containing two thousand answers to curious questions in most arts and sciences, serious, comical, and humorous, approved of by many of the most learned and ingenious of both universities, and of the Royal-Society‎, 3rd edition, volume 1, the Bell in Little Britain: printed for Theodore Sanders, page 67