I have a name dictionary (Dunkling 1983) that says this:
"Earliest use of this name was for men in Scotland and the north of England. Represented a diminutive of Christopher, and frequently appeared as Chrystal. This name itself had a diminutive Christie, or Christy. The feminine name, which has been used intermittently since the beginning of the century [20th], is a transferred use of the word 'crystal', with its suggestion of brightness and clarity, ultimately from Greek krystallos 'ice'."