Agatized Coral
Chalcedony pseudomorph after Anthozoa · Collector Mineral and Semi-Precious Gemstone
Crystal System
Trigonal (microcrystalline/cryptocrystalline); usually occurs in massive or botryoidal forms within the original skeletal structure.
Mineral Group
Silicates (Subgroup: Tectosilicates / Silica group)
Chemical Formula
SiO2 (Silicon Dioxide with trace inclusions of iron, manganese, and calcium)
Formation Process
Metasomatism/Silicification: A sedimentary process where silica-rich groundwater replaces the original calcium carbonate skeleton of ancient coral over millions of years.
About This Crystal
An opaque to translucent tan/brown specimen with a waxy to vitreous luster. The surface is characterized by numerous small pits or vugs, likely where delicate skeletal structures were not fully replaced or have weathered away. It shows a water-worn, tumbled texture.
Physical Characteristics
Color: Earthy tan, brown, and grey; Streak: White; Cleavage: None; Fracture: Conchoidal to uneven; Specific Gravity: 2.60–2.65; Fluorescence: Often fluoresces yellow or green under LWUV if containing specific trace minerals.
Optical Properties
Refractive Index: 1.530–1.543; Birefringence: Up to 0.004; Pleochroism: None; Optical phenomena: Can occasionally show 'star' or 'flower' patterns (agatized polyps).
Hardness & Durability
Origin Region
Prominently found in Florida (Tampa Bay/Withlacoochee River), Georgia, and Indonesia (West Java). Forms in marine limestone environments.
Hardness & Durability
Mohs Hardness: 6.5 to 7.0; Toughness: Very high due to cryptocrystalline structure; Stability: Robust, though pitted surface can trap debris.
Care & Maintenance
Safe to clean with warm soapy water and a soft brush. Avoid harsh chemicals that could react with any residual calcite; ultrasonic cleaners are generally safe but use caution with highly porous specimens.
Rarity & Value
Relatively common but high-quality pieces with visible 'flower' patterns are more valuable. In Florida, it is the state rock. Values range from a few dollars for rough to $50+ for polished display pieces.
Special Characteristics
As a pseudomorph, it retains the biological anatomy of the original coral while being entirely composed of quartz. Some Indonesian varieties exhibit vibrant colors due to trace mineral contamination.
Lore & History
Historically used by Indigenous Floridians for projectile points and tools. Metaphysically, it is traditionally associated with grounding, transition, and connection to the Earth's history and ancient wisdom.