Agate
Quartz var. Agate · Semi-Precious Gemstone and Mineral Specimen
Crystal System
Trigonal, cryptocrystalline microfibrous variety of quartz, habit typically forms in concentric or layered nodules (geodes or amygdales) within volcanic and metamorphic rocks.
Mineral Group
Silicates (Tectosilicates)
Chemical Formula
SiO2
Formation Process
Hydrothermal and sedimentary deposition involving silica-rich groundwater or hydrothermal solutions filling cavities in pre-existing rocks in low-temperature, low-pressure environments over extended periods.
About This Crystal
This specimen is a polished flat cross-section slice of a nodule, displaying a distinctive outer ring of brownish-purple or mauve microcrystalline quartz enclosing a contrasting central cavity filled with banded white and translucent chalcedony layers. The surface exhibits a smooth polished finish with a vitreous to waxy luster, displaying concentric zoning and deposition lines.
Physical Characteristics
Color varies from brownish-purple to white and translucent gray; white streak; no true cleavage (conchoidal fracture); specific gravity is approximately 2.60 to 2.65; brittle tenacity; displays no significant magnetism, but exhibits piezoelectric properties.
Optical Properties
Subtranslucent to opaque in the outer zone and translucent in the center; singly or aggregate doubly refractive with a refractive index of approximately 1.544 to 1.553; no strong pleochroism due to cryptocrystalline nature.
Hardness & Durability
Origin Region
Found worldwide, with notable deposits in Brazil, Uruguay, India, Madagascar, Germany, and the United States (such as Oregon and Lake Superior regions). Typically forms in volcanic cavities and gas vesicles in basaltic lavas.
Hardness & Durability
Mohs hardness of 6.5 to 7; good durability and toughness due to interlocking microcrystalline structure, making it suitable for general jewelry and ornamental objects; compares to quartz.
Care & Maintenance
Easy to care for; clean with warm soapy water and a soft brush; avoid harsh chemical cleaners and sudden extreme temperature changes; store away from prolonged direct sunlight to prevent potential color fading if dyed.
Rarity & Value
Common; widespread and accessible, though high-quality specimens with exceptional color contrast, unique patterns, or large sizes can command higher prices. Common treatments include dyeing to enhance or alter colors.
Special Characteristics
Banded deposition structures, often showing distinct levels of silica precipitation, fluid inclusions, or crystal growth stages within a hollow geode core.
Lore & History
Historically used by ancient civilizations such as the Egyptians, Greeks, and Romans for amulets, seals, and ornamental vessels to ward off stress and promote emotional balance. In modern crystal healing, it is traditionally associated with grounding, stability, and balancing physical energy, though these are cultural beliefs rather than scientific claims.