Selenite

Selenite is a crystalline variety of gypsum that forms through the evaporation of calcium sulfate-rich water. In its most dramatic expression, this process created the Giant Crystal Cave (Cueva de los Cristales) in Naica, Mexico, where selenite crystals grew to 12 meters long and weighed up to 55 tons - the largest crystals ever discovered on Earth.

The Naica crystals grew over approximately 500,000 years in a cave filled with mineral-rich water heated to about 58 degrees Celsius by underlying magma. At this precise temperature, gypsum is barely soluble - meaning crystals could grow extremely slowly, adding only a few atomic layers per year. This glacial growth rate, combined with perfectly stable conditions over millennia, produced crystals of extraordinary size and perfection.

Desert rose selenite forms through a different process - gypsum crystallizes within sand in arid environments, incorporating sand grains as it grows and forming rosette-shaped clusters. Oklahoma's hourglass selenite contains an internal hourglass-shaped pattern of brown sand trapped during growth, visible when the crystal is held up to light.

Selenite is instantly recognizable by its extreme softness. At Mohs 2, you can scratch it with your fingernail. It's one of the softest minerals you'll commonly encounter. Transparent selenite transmits light beautifully and has a distinctive pearly to vitreous luster.

The crystal habit is typically bladed (flat, sword-like crystals) or tabular. Selenite has perfect cleavage in one direction, meaning thin sheets can be peeled off with a fingernail. A critical property to know: selenite is water-soluble. Never clean selenite with water or it will slowly dissolve and lose its polish. This same property means selenite left outdoors will deteriorate.