Chiastolite
Andalusite var. Chiastolite · Collector Mineral / Semi-Precious Gemstone
Crystal System
Orthorhombic. Typically forms prismatic crystals with a nearly square cross-section, but is most famous for its characteristic internal cross pattern formed by carbonaceous inclusions.
Mineral Group
Silicates (Nesosilicates)
Chemical Formula
Al2SiO5
Formation Process
Regional and contact metamorphism of argillaceous (clay-rich) sediments at low to moderate pressures and temperatures, where carbonaceous matter is systematically trapped along the growing crystal's crystallographic axes.
About This Crystal
An opaque, brownish to greyish mineral specimen notable for the distinct, dark cross-shaped pattern clearly visible on its cross-section surface, resulting from symmetrically arranged graphite inclusions.
Physical Characteristics
Color is greyish-white, yellowish, or brown with a characteristic black or dark brown cross. Streak is white. Cleavage is distinct on {110}. Fracture is uneven to subconchoidal. Specific gravity is approximately 3.13 to 3.16. Tenacity is brittle.
Optical Properties
Biaxial negative with moderate to high relief. Refractive indices range from 1.629 to 1.643. Exhibits weak to moderate pleochroism in transparent gem varieties, though chiastolite specifically is typically opaque.
Hardness & Durability
Origin Region
Notable sources include Spain, Russia, Australia, China, and the USA (California). Forms in aluminous metamorphic rocks.
Hardness & Durability
Mohs hardness of 6.5 to 7.5 on the outer andalusite zones, but can be softer along the carbon-rich inclusion paths. Moderate durability; care should be taken to avoid hard impacts due to distinct cleavage.
Care & Maintenance
Clean gently with warm soapy water and a soft brush. Avoid ultrasonic cleaners, harsh chemicals, and sudden thermal shocks.
Rarity & Value
Uncommon to rare collector's mineral. Value depends primarily on the symmetry, clarity, and aesthetic perfection of the internal cross pattern, as well as the overall size of the specimen.
Special Characteristics
The defining special characteristic is the naturally occurring cruciform (cross) inclusion pattern formed by graphite or carbonaceous impurities during crystal growth.
Lore & History
Historically known as the 'Cross Stone', chiastolite was often worn or carried as a natural amulet or talisman for protection, grounding, and connection to ancient earth energies. In crystal healing, it is traditionally associated with the root chakra to calm fears and enhance problem-solving.