Gypsum and selenite are chemically similar because they are both composed primarily of calcium sulfate. The difference between the two minerals lies in their crystalline structure and water content. Gypsum is a hydrated calcium sulfate mineral with the chemical formula CaSO4·2H2O, meaning that it contains two molecules of water for each molecule of calcium sulfate. Selenite, on the other hand, is an anhydrous calcium sulfate mineral with the chemical formula CaSO4, indicating that it does not contain any water molecules.

Despite their different crystalline structures and water content, gypsum and selenite share similar chemical properties. Both minerals are relatively soft, with a Mohs hardness of 2, and have a white or colorless appearance. They are also both soluble in water and have a similar specific gravity.