Agate
Quartz var. Agate (Microcrystalline Chalcedony) · Semi-Precious Gemstone / Mineral Specimen
Crystal System
Trigonal/Monoclinic (Microcrystalline); typical habit is massive, banded, or nodular fillings
Mineral Group
Silicates (Tectosilicate)
Chemical Formula
SiO2 (Silicon Dioxide) with traces of Iron, Alumina, and Manganese
Formation Process
Hydrothermal; formed by the deposition of silica-rich groundwaters in rock cavities (geodes or vesicles), creating layered bands.
About This Crystal
The specimen shows dark host rock or massive chalcedony with distinct, translucent blue-gray banding. It exhibits a vitreous to waxy luster and is opaque to translucent on the edges.
Physical Characteristics
Color: Dark grey/brown with blue-white bands; Streak: White; Fracture: Conchoidal; Specific Gravity: 2.60–2.65; Fluorescence: Often inert but can show weak green or yellow under UV.
Optical Properties
Refractive Index: 1.530–1.540; Birefringence: Up to 0.009; Optical Character: Uniaxial positive; Lacks significant pleochroism.
Hardness & Durability
Origin Region
Brazil, Uruguay, Oregon (USA), and India are major sources. Found in volcanic basalt and sedimentary cavities.
Hardness & Durability
Mohs Hardness: 6.5–7; highly durable and tough due to its cryptocrystalline structure, making it excellent for jewelry.
Care & Maintenance
Clean with warm soapy water and a soft brush. While stable, avoid extreme heat and harsh acids. Resistant to most household chemicals.
Rarity & Value
Common; value depends on the uniqueness of the banding patterns and color. Commercial grade specimens are very affordable ($5-$20 for raw pieces).
Special Characteristics
Exhibits classic 'fortification' banding; some specimens may show botryoidal growth or druzy quartz centers.
Lore & History
Used since the Bronze Age in amulets and hardstone carvings. Traditionally associated with grounding, balance, and protection; historically believed to quench thirst and protect against fevers.