Petrified (Coral/Shell)
Petrified (Coral/Shell) is a type of material that has fossilized as living tissue is replaced by silica (SiO2). In some cases, the silica merely replicates the external form of the organism; this fossilization process is known as petrification.
A fossil material in which the original tissue is preserved as a pattern after being replaced by silica.
Basic Information
| English Name | Petrified Coral/Shell |
|---|---|
| Japanese Name | Keika Sango (Keika Kai) |
| Chemical Formula | SiO2 + Impurities |
| Classification | Fossil (Silicified biological tissue) |
| Hardness | 6–7 (for silicified fossils) |
| Specific Gravity | — |
| Transparency | — |
Characteristics & Variations
- Type Classification: Fossils primarily composed of SiO2 include Agatized (structures like Agate), Silicified (no agate-like structure), and Opalized (hydrated states comparable to Opal).
- Colors: Generally grey or white, while orange, purple, and especially jet-black varieties are rare.
- Valuation: Items are more highly valued if they are vibrant and beautiful, and if the original biological tissue is clearly preserved.
- Patterns: Petrified coral often exhibits uniform radial patterns, while petrified shells may show crescent-shaped cross-sections in bivalves, or bamboo-shoot-like cross-sections in gastropods.
- Varieties: Enhydrous petrified shell (sometimes water is trapped inside the petrified snail shell).
- Note: In places like Australia, shell fossils that have turned into precious opal (black opal) are famous.
Treatments & Related Info
Treatment: Recently, some items, especially beads, have been treated with synthetic resin impregnation.
Related Info: "Kikuseki" (Chrysanthemum stone): The geometric patterns created by coral tissue structures resemble chrysanthemum flowers, which is why they are also called "Chrysanthemum stone" or "Chrysanthemum pattern stone" in the world of suiseki (viewing stones).
Caution: Red-colored solitary coral fossils are produced in Utah, USA; this red color is due to the inclusion of iron oxide colloids (hematite), which are sensitive to UV rays and light. Avoid long-term direct sunlight.
Note: If the items (such as beads) have been impregnated, cleaning or the use of solvents may alter the surface condition. Also, avoid prolonged exposure to direct sunlight.