Agate
Chalcedony (cryptocrystalline variety of Quartz) · Semi-Precious Gemstone / Decorative Stone
Crystal System
Trigonal (microcrystalline/cryptocrystalline), typically occurs in massive or nodular habits with distinct banding
Mineral Group
Silicates (Tectosilicate)
Chemical Formula
SiO2 (Silicon Dioxide) with Fe2O3 (Hematite) inclusions
Formation Process
Hydrothermal; formed by the deposition of silica from groundwater in the cavities of igneous rocks or volcanic vesicles
About This Crystal
An opaque to translucent cryptocrystalline specimen showing earthy reddish-brown and grey tones with a distinct waxy to vitreous luster on fractured surfaces and a white chalcedony band or vein
Physical Characteristics
Color: Red-brown, grey, white; Streak: White; Cleavage: None; Fracture: Conchoidal to uneven; Specific Gravity: 2.60-2.65; Fluorescence: Usually inert or faint yellow-green
Optical Properties
Refractive Index: 1.530-1.543; Birefringence: up to 0.004; Optical character: Uniaxial positive; No pleochroism
Hardness & Durability
Origin Region
Brazil, India, Uruguay, and Lake Superior region (USA); formed in volcanic host rocks and sedimentary cavities
Hardness & Durability
Mohs Hardness: 6.5 to 7; High toughness due to interlocking microcrystalline structure; excellent durability for jewelry
Care & Maintenance
Clean with warm soapy water; avoid harsh chemicals and prolonged exposure to high heat which may alter the iron-based color
Rarity & Value
Common; Value is determined by pattern clarity, color contrast, and size; often inexpensive but highly collected for unique patterns
Special Characteristics
Displays micro-banding (agatic banding) and inclusions of iron oxides that produce the 'mossy' or mottled reddish appearance
Lore & History
Historically used for amulets and hardstone carving in Ancient Greece and Rome; traditionally associated with grounding, balance, and physical protection.