Selenite

Hardness: 2 (very soft on the Mohs scale, can be scratched with a fingernail). Color: Typically clear, colorless, or white; can be tinted by impurities (e.g., orange, brown, green). Luster: Pearly to vitreous (glassy). Crystal Structure: Monoclinic, often forming tabular, prismatic, or acicular crystals. Exhibits excellent cleavage in one direction, yielding thin, flexible sheets.

Selenite is a variety of the mineral gypsum, which forms typically from the evaporation of saline water (e.g., ancient seas, salt lakes, or caves) under arid conditions. It is a common evaporite mineral and can form in various sedimentary environments. Its geological age varies widely depending on the specific deposit, ranging from relatively recent to millions of years old.

Industry: Used in plaster, drywall, cement, and as a fertilizer. Construction: As an architectural stone, though less common due to its softness. Jewelry: Sometimes cut into cabochons or beads, but its softness makes it prone to scratching. Collecting: Highly valued by mineral collectors for its unique crystal forms, clarity, and optical properties (e.g., 'desert rose' forms, 'fishtail' twins, and 'satin spar' fibrous varieties).

The name 'selenite' comes from the Greek word 'selene', meaning moon, referring to the moon-like glow or pearly luster of some specimens. It is known for its perfect cleavage, allowing it to be split into thin, flexible, transparent sheets that were historically used as window panes (especially in ancient Rome, called 'Lapis Specularis'). Selenite can also form in massive, fibrous (satin spar), or rosette-like (desert rose) habits. It is one of the softest minerals.

Identification in the field: Selenite is easy to identify due to its extreme softness (scratches with a fingernail), excellent cleavage into thin, flexible sheets, and often transparent to translucent appearance. It frequently occurs in association with other evaporite minerals like halite, anhydrite, and sulfur. Common locations: Major deposits are found worldwide, including Naica, Chihuahua, Mexico (famous for giant selenite crystals); the Great Salt Plains, Oklahoma, USA; and various locations in Spain, Australia, and Canada. Tips for collectors: Handle with care due to its softness; avoid prolonged exposure to water as it can dissolve. Clean with a dry cloth or gentle brushing.