Satin Spar (often commercially called Selenite)
Gypsum var. Satin Spar · Collector Mineral / Decorative Stone / Lapidary Material
Crystal System
Monoclinic; typically occurs in fibrous, silky, massive aggregates as shown in the specimen.
Mineral Group
Sulfates (Hydrated sulfates)
Chemical Formula
CaSO4·2H2O (Hydrous calcium sulfate)
Formation Process
Sedimentary process through the evaporation of saline water (evaporite) in alkaline lake beds or clay beds.
About This Crystal
The specimen shows a pearly to silky luster with a distinctive fibrous structure. It is translucent white with parallel internal fibers that create a 'cat’s eye' or chatoyant effect.
Physical Characteristics
Color: White/Colorless; Streak: White; Cleavage: Perfect in one direction (micaceous); Fracture: Splintery/Fibrous; Specific Gravity: 2.3; Fluorescence: Can be weak orange or blue under UV.
Optical Properties
Refractive Index: 1.520-1.530; Birefringence: 0.009; Pleochroism: None; Optical Phenomena: Chatoyancy (the 'silk' effect) due to its fibrous nature.
Hardness & Durability
Origin Region
Morocco (major commercial source), Mexico (Naica), USA (Utah), and various evaporite basins worldwide.
Hardness & Durability
Mohs Hardness: 2. Extremely soft (can be scratched by a fingernail). Poor toughness; very fragile and easily crushed or scratched.
Care & Maintenance
Water soluble; do not soak. Avoid all chemicals, ultrasonics, and steam. Store separately to prevent scratches from harder minerals. Clean only with a soft, dry cloth.
Rarity & Value
Very Common. Primarily valued for its aesthetic fibrous luster. Inexpensive, typically sold by weight/size in rock shops.
Special Characteristics
Strong chatoyancy (fiber-optic effect) where light reflects off the internal parallel fibers. It is also a natural thermal insulator.
Lore & History
Historically used as window panes (as Lapis Specularis) in ancient Rome. In modern lore, it is believed to be 'liquid light' used for cleansing other crystals and the Crown Chakra.