Alabaster is a mineral and a soft rock used for carvings and as a source of plaster powder. Archaeologists, geologists, and the stone industry have different definitions and usages for the word alabaster. In archaeology, the term alabaster is a category of objects and artefacts made from the varieties of two different minerals: (i) the fine-grained, massive type of gypsum, and (ii) the fine-grained, banded type of calcite.
Calcite alabaster: The tomb of Tutankhamun (d. 1323 BC) contained a practical objet d’art, a cosmetics jar made of Egyptian alabaster, which features a lid surmounted by a lioness (goddess Bast).
Alabaster artefact: A composite bust of the Emperor Septimius Severus; the head is marble and the bust is alabaster.
Alabaster windows in the Church of Santa Maria la Mayor of Morella, Spain (built 13th-16th centuries)
Alabaster workshop in Volterra, Italy
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)
