Selenite is a crystalline variety of the mineral gypsum, a hydrous calcium sulfate. While it is widely known for its bright, milky white appearance, the purest form of selenite is actually transparent and colorless. The name selenite is derived from the Greek word for “moon,” alluding to the soft, pale glow the mineral often exhibits. Its color and transparency vary dramatically depending on its chemical purity and the specific structure in which the crystals formed.
The Purest Form: Why Selenite Is Colorless
The chemical composition of selenite determines its baseline lack of color. In its chemically pure state, selenite is Calcium Sulfate Dihydrate. This arrangement creates a lattice structure that does not contain chromophores, which are ions or molecular groups that absorb certain wavelengths of light.
Because the pure crystal structure does not absorb visible light, light waves pass through the crystal unimpeded. This lack of absorption results in a perfectly transparent or clear appearance, much like glass. Transparent varieties are often found as large, well-formed crystals with a vitreous luster, leading to its historical use as window panes.
Mineral Impurities That Create Color Variations
The vibrant colors seen in many selenite specimens are not a feature of the pure mineral but are caused by the inclusion of foreign materials during crystallization. Trace elements and other mineral particles become trapped within the gypsum structure as the crystal grows, acting as pigments. The final hue and its intensity depend directly on the type and concentration of the impurity present.
Orange and red selenite is primarily colored by the presence of iron oxides. These iron-containing compounds, often introduced through groundwater, tint the crystal with warm shades. Other foreign substances, such as clay, sand, or organic matter, are responsible for gray, brown, or yellow varieties. Rarer green selenite occurs when the crystal incorporates trace amounts of copper-based minerals.
The most common secondary color, a translucent white, is caused by microscopic inclusions or imperfections that scatter light. This scattering effect creates the pearly or milky appearance often associated with “white selenite.” Even a small amount of a contaminant can shift the crystal’s color from colorless to a pale tint.
How Crystal Structure Affects Appearance
Beyond the chemical impurities that supply the pigment, the physical way the selenite crystals form—known as their crystal habit—radically alters their visual appearance. The term selenite is often used broadly to describe three distinct habits: transparent selenite, Satin Spar, and Desert Rose.
Transparent selenite forms in large, tabular sheets or prismatic crystals with high clarity. In contrast, Satin Spar consists of densely packed, parallel, fibrous crystals. This fibrous structure reflects and scatters light along the internal fibers, producing a characteristic silky luster and an optical effect called chatoyancy, or the “cat’s eye” sheen.
Another distinct habit is the Desert Rose, which forms as rosette-like clusters where individual selenite blades are heavily mixed with sand and clay particles. The high concentration of non-gypsum material and the clustered structure make the mineral opaque, giving it a sandy, brown or tan coloration and texture. The final look of selenite is a combination of its base color and its unique physical crystal structure.