Definition of "chrystal"

Conſider there how black and fowl your Sin / Is rendred by my Chryſtal innocence: […]

c. 1635–39?, Richard Brome, The Queen and Concubine. A Comedie, London: […] A. Crook, and Hen. Brome, […], published 1659, act IV, scene III, page 81

We had one heavy shower on leaving Sittingbourne, but afterwards the clouds cleared away, and we had a very bright chrystal afternoon.

1798 October 24, Jane Austen, “Letter VIII”, in Edward, Lord Brabourne, editor, Letters of Jane Austen, volume I, London: Richard Bentley & Son, […], published 1884, pages 155–156

If we weep by Babel’s streams, the sparkle of our chrystal tears, touch the beam which kindles from above; […]

1848, The Dirge; or, A Voice in the Night. Originally Addressed to a Clergyman at Edinburgh, 1845., Edinburgh: […] Anderson and Bryce, […], page 575