Agate
Quartz var. Agate (IMA-approved mineral: Quartz) · Semi-Precious Gemstone / Mineral Specimen
Crystal System
Trigonal crystal system; microcrystalline (cryptocrystalline) structure. It typically occurs in massive, nodular, or amygdaloidal habits.
Mineral Group
Silicates (Subgroup: Tectosilicates)
Chemical Formula
SiO2 (Silicon Dioxide) with micro-inclusions of iron oxides (hemitite/goethite) providing the brown/tan coloration
Formation Process
Hydrothermal/Sedimentary process: Silica-rich groundwater percolates into cavities in volcanic rock, depositing layers of microcrystalline quartz over time from the outer walls inward.
About This Crystal
A polished, tumbled specimen showing distinctive concentric banding (fortification agate). Colors range from milky white and translucent gray to earthy tan and reddish-brown. It has a vitreous (glassy) luster when polished and is translucent to opaque.
Physical Characteristics
Color: Multi-colored bands; Streak: White; Cleavage: None (cryptocrystalline); Fracture: Conchoidal; Specific Gravity: 2.58–2.64; Fluorescence: Often inert, but may show dull green or white under LWUV depending on trace elements.
Optical Properties
Refractive Index: 1.544–1.553; Birefringence: Weak (0.009); Pleochroism: None; Optical Phenomena: Banding (distinctive layers of varying translucency).
Hardness & Durability
Origin Region
Globally distributed; notable sources include Brazil, Uruguay, Botswana, and Lake Superior (USA). Formed typically in volcanic cavities (vesicles).
Hardness & Durability
Mohs Hardness: 6.5–7. It is very tough and stable, making it excellent for jewelry and decorative items. It is resistant to scratch damage from most household objects.
Care & Maintenance
Wash with warm soapy water and a soft brush. Avoid ultrasonic cleaners if the stone has dyed treatments or surface-reaching fractures. Generally resistant to sunlight, but avoid harsh chemicals.
Rarity & Value
Rarity: Common. Value is determined by the uniqueness of the banding pattern and color intensity. Tumbled stones like this are very affordable, typically $1-$10 depending on size.
Special Characteristics
Fortification banding: The appearance of concentric layers that resemble the layout of an old fort. The specimen shows a clear boundary between the milky white matrix and the pigmented quartz layers.
Lore & History
Ancient Greeks and Egyptians used Agate for amulets and seals. It was historically believed to provide protection and cure stings from scorpions. In modern metaphysical practices, it is associated with grounding, balance, and the Root Chakra.