The Collaborative International Dictionary of English v.0.48:

Prian \Pri"an\, n. [Cornish, clayey ground, from pri clay.]
   (Mining)
   A fine, white, somewhat friable clay; also, the ore contained
   in a mixture of clay and pebbles. [Written also pryan.]
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Pryan \Pry"an\, n. (Mining)
   See Prian.
   [1913 Webster]