Merriam-Webster, s. v. “crystal (n.),” accessed March 23, 2021, https://www.merriam-webster.com/dictionary/crystal.
Chamberlain, Wicca Crystal Magic, chap. 1.
Cunningham, Cunningham’s Encyclopedia of Crystal, Gem & Metal Magic, 1.
Studeman, “Are You Obsessed with Crystals, Too?”