Selenite is a common mineral, a crystalline variety of gypsum (hydrated calcium sulfate, CaSO₄·2H₂O). It is known for its exceptional clarity and is often used in decorative and spiritual items. The name “selenite” derives from the Greek word for Moon, selēnē, referencing the soft light it transmits. This mineral forms in evaporite environments where ancient saline water bodies slowly dry out.
Primary Visual Characteristics
Selenite is most often colorless and transparent. This clarity is a defining visual trait, though it can also range to a milky, translucent white. When impurities are present in its crystalline structure, selenite can take on a variety of pale colors. These tints may include light yellow, orange, or brown, and more rarely, shades of blue or green.
The surface of selenite exhibits a characteristic luster that helps identify it. The mineral typically displays a vitreous, or glass-like, shine on its main surfaces. However, on surfaces created by cleavage, the luster often appears pearly.
Diverse Crystal Habits and Forms
The appearance of selenite is heavily influenced by its crystal habit, the characteristic shape in which it grows. The classic “selenite” form is found as large, tabular crystals that are typically clear and plate-like. These crystals can grow to enormous sizes, such as the massive specimens found in the Cave of the Crystals in Mexico. The transparent nature of these tabular crystals historically allowed them to be used as window panes.
Satin Spar
Another common form is Satin Spar, which fundamentally differs in appearance from the clear variety. This form is composed of parallel, fibrous crystals bundled together, giving it a translucent, silky white or peach color. When polished, Satin Spar exhibits a visual phenomenon called chatoyancy, or the “cat’s eye” effect, where a band of light appears to move across the surface.
Desert Rose
The third distinct habit is the Desert Rose, an intricate cluster of selenite crystals intergrown with sand or clay. This intergrowth causes the crystals to form a bladed, rosette-like structure that visually resembles a flower. Desert Roses are often opaque due to the sand inclusions and typically have a duller luster.
Defining Physical Properties
The most notable characteristic is its extreme softness, registering only a 2 on the Mohs scale of mineral hardness. This low hardness means the mineral can be easily scratched or marked with a fingernail. This softness makes selenite easy to carve into decorative objects like bowls, though it also requires careful handling to prevent damage.
Selenite also possesses perfect cleavage in one direction, meaning it splits readily along distinct, flat planes. This micaceous cleavage allows the mineral to be separated into thin, flexible sheets. The presence of water molecules in its chemical structure makes it a hydrous calcium sulfate, contributing to both its softness and its unique thermal properties.
Unlike most minerals, selenite has natural thermal insulating properties, meaning it does not conduct heat well. Consequently, a piece of selenite often feels noticeably warmer to the touch than other minerals or rocks at the same temperature.