A desert rose is an intricate rose-like formation of crystal clusters of gypsum or baryte, which include abundant sand grains. The "petals" are crystals flattened on the c axis, fanning open in radiating clusters.
Saharan gypsum desert rose from Tunisia (length 47 cm)
Baryte rose from Cleveland County, Oklahoma (size: 10.2 × 7.1 × 5.5 cm)
Large desert rose formation in the Tunisian desert
Gypsum is a soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula CaSO4·2H2O. It is widely mined and is used as a fertilizer and as the main constituent in many forms of plaster, drywall and blackboard or sidewalk chalk. Gypsum also crystallizes as translucent crystals of selenite. It forms as an evaporite mineral and as a hydration product of anhydrite. The Mohs scale of mineral hardness defines gypsum as hardness value 2 based on scratch hardness comparison.
Gypsum
Gypsum crystals are soft enough to bend under pressure of the hand. Sample on display at Musée cantonal de géologie de Lausanne.
Large gypsum crystals in Lechuguilla Cave's "chandelier ballroom"
Gypsum crystals in the Cave of the Crystals in Mexico (person at lower right for scale)