Selenite and gypsum are often confused because they share a close relationship, but understanding their differences can help collectors, enthusiasts, and professionals appreciate their unique qualities.

Naica selenite crystals are a variety of gypsum, but they differ significantly from typical gypsum crystals due to their extraordinary size and clarity. Some of these crystals reach lengths of up to 12 meters (about 40 feet), making them among the largest natural crystals ever discovered.