Gypsum is one of the most common and widely used minerals on Earth, renowned for its softness, versatility, and beauty. Chemically, it is a hydrated calcium sulphate – CaSO₄·2H₂O – formed by the evaporation of saline waters in hot sedimentary environments such as lake beds, lagoons, and marine basins. With a hardness of just 2 on the Mohs scale, it can easily be scratched with a fingernail, while its perfect cleavage and low specific gravity distinguish it from other minerals of similar appearance.
Crystallising in the monoclinic system, gypsum often forms tabular, prismatic, or fibrous crystals. Its structure consists of layers of calcium and sulfate ions bound together with water molecules, a combination which gives the mineral its characteristic softness and pearly lustre. This internal arrangement allows gypsum to lose or regain water with ease, transforming into anhydrite – CaSO₄ – when dehydrated and rehydrating back into gypsum under suitable conditions. This reversible process has fascinated geologists for centuries and contributes to the mineral’s widespread occurrence in nature and use in industry.
Gypsum occurs in several distinct varieties, each notable for its unique appearance and texture.
Selenite – Transparent and colourless crystals that sometimes form large sheets, often called 'windowpane' crystals. This is one of the most common and visually striking forms of gypsum.
Satin Spar – An opaque, silky, fibrous variety that often exhibits a beautiful chatoyant effect, similar to the shimmer seen in cat’s-eye gemstones.
Alabaster – A fine-grained, massive form of gypsum valued since ancient times for its softness and smooth polish, making it a favourite material for carving sculptures and decorative objects.
Ram’s Horn – A distinctive type of satin spar that has grown unevenly, producing gracefully curved crystals resembling animal horns.
Desert Rose - A rosette-shaped formation of gypsum crystals that develop in arid, sandy environments. The crystals trap sand grains as they grow, giving the cluster its distinctive petal-like appearance and warm, earthy colour.
Daisywheel - Radiating crystal formations of gypsum where flat, bladed crystals spread outward from a central point on matrix, resembling the petals of a daisy flower.
Fishtail - A twinned crystal form of gypsum where two flat blades diverge from a common base, creating a shape that closely resembles a fish’s tail or a swallow’s wings.
Deposits of gypsum are found around the world, often in impressive abundance. One of the most famous occurrences is the Naica Mine in Chihuahua, Mexico, home to the astonishing 'Cave of the Crystals', where selenite crystals reach lengths of more than ten metres – some of the largest crystals of any mineral ever discovered. Sicily has been a historic centre of gypsum mining since Roman times, while Nova Scotia in Canada provides fine alabaster used in decorative arts. In the United States, large deposits are exploited in Oklahoma and Texas for industrial production, and England’s Nottinghamshire and Spain’s Sorbas Desert are celebrated for their beautiful satin spar and fibrous gypsum veins.
Few minerals rival gypsum in economic importance. When heated to about 150°C, it loses part of its water and becomes plaster of Paris, a material that hardens when mixed with water. This property has been known since ancient Egyptian and Roman times and remains the foundation of many modern industries. Gypsum is essential in the production of wallboard and plaster for construction, in cement manufacturing where it acts as a setting retardant, and in agriculture where it improves soil structure and provides calcium and sulfur nutrients. It is also used in ceramics and dentistry for moulds and casts.
Beyond its industrial significance, gypsum offers several intriguing natural stories. The colossal crystals in Mexico’s Cave of the Crystals formed over half a million years under stable temperatures of around 58°C, growing slowly in mineral-rich water to produce their extraordinary size. The mineral’s name itself comes from the Greek word gypsos, meaning 'plaster', a testament to its long association with human craftsmanship. Even nature’s landscapes have been shaped by gypsum, as seen in White Sands National Park in New Mexico, where vast sparkling dunes of pure gypsum sand stretch across the desert – the only such formation of its kind on Earth.
From delicate crystals to monumental industrial use, gypsum bridges the gap between natural beauty and human utility. Its softness and fragility conceal a remarkable resilience, making it one of the world’s most enduring and indispensable minerals – a substance that has shaped both landscapes and civilisations throughout history.
The Assay House—Mineral Dealers of quality Mineral Specimens
Image Credits - CC Robert M. Lavinsky & James St. John