Definition of "porphyre"

Let us consider the red and white colours in porphyre: hinder light but from striking on it, and its colours vanish; it no longer produces any such ideas in us […]

1689 (indicated as 1690), [John Locke], chapter 2, in An Essay Concerning Humane Understanding. […], London: […] Eliz[abeth] Holt, for Thomas Basset, […], book I, page 8