Fossil Coral

Fossil Coral Gemstone Information

Fossil coral is a natural type of gemstone formed by ancient corals. Corals are made up of small invertebrate animals, known as zooids, that look like tiny sea anemones. They feed on small food particles they find in the water around them. The proper name for fossil coral is ‘agatized coral’ or ‘agatized fossil coral’, because, during formation, the coral remains are gradually replaced with agate, a variety of naturally occurring chalcedony, or microcrystalline quartz. When prehistoric corals are fossilized through replacement with agate, the fossil coral forms through hardened deposits left by silica-rich waters.

The entire process can take over 20 million years and occurs only under very unique geological conditions. Corals are marine animals and it is their skeletons that are fossilized and preserved, often leaving flower-like patterns in the stone.
The oldest coral fossils are over 500 million years old. The earliest forms were different from those we see today and they died out 225 million years ago. Modern corals are still common in tropical oceans.

Fossil coral should not be mistaken for endangered or protected reef coral or precious coral. It is considered to be a type of agate or chalcedony, rather than a type of coral, due to its silicon dioxide composition (SiO2). Coral deposits have been mined and commercialized for their high calcium, potassium, magnesium, and sodium content, and are often used for the making of health and drug supplements. Fossil coral is used in industrial fertilizers and water purification filters because it can remove chemical impurities such as chlorine and formaldehyde.

Identifying Fossil Coral 

Fossil coral is created through the process of replacement, whereby calcium carbonate (coral) is replaced by silica. This unique process of preservation can result in different mineral concentrations from specimen to specimen. Silica can range from light amorphous opaline to dense forms of chalcedony. Thus, fossil coral specimens may have slightly varying densities depending on the exact mineral concentration. Fossil coral can be mistaken for other types of agate such as moss or dendritic agate, but the pattern and body color of fossil coral is quite distinct. Fossil coral is also much harder than precious coral. Most agatized fossil coral exhibits a dull to a waxy luster and interesting skeletal-like ancient coral patterns, most often appearing in flower shapes.

Fossil Coral Fossil Coral

Fossil Coral Origin and Gemstone Sources

Almost all fossil coral specimens are found in either Indonesia or the United States (Florida and Georgia). Florida has even declared agatized fossil coral as their official ‘Florida State Rock’. Indonesian fossilized coral is considered to be the most desirable and unique. In Indonesia, entire coral heads are often found perfectly preserved, appearing exactly as they did 20 million years ago, though density may vary according to the exact chemical replacement of silica, manganese, iron and other various minerals.

Buying Fossil Coral and Determining Value

Fossil Coral Color

Agatized fossil coral can exhibit a wide range of natural colors, ranging from white and pink, to brown, gray, black, yellow and red. Coral color may vary widely within a single stone, depending on weathering, oxidation and the original mineral content.

Fossil Coral Fossil Coral

Fossil Coral Clarity and Luster

Fossil coral is typically opaque, but some specimens may appear near-translucent. When cut and polished, they have a waxy to dull luster.

Fossil Coral Cut and Shape

Fossil coral is often shaped and polished as cabochons. They are frequently used for ornamental gemstone carvings. Most cabochons are cut with low domes or flat surfaces. The most common shapes are ovals, rounds and cushions. Fancy and freeform shapes are also quite popular. Now you can find accessories made of coral fossils in Smartyleowl.

Fossil Coral Fossil Coral

Fossil Coral Treatment

Fossil coral is not typically treated or enhanced in any way. However, there have been reports of heated fossil coral making its way to the markets. The heating is a form of enhancement treatment used to produce stronger reddish hues in specimens. The treatment is very hard to detect; most labs are unable to detect such mild heating.

Fossil Coral Gemological Properties:

Chemical Formula: SiO2; Silicon dioxide
Crystal Structure: Hexagonal; hexagonal prisms
Color: Mostly gray; all colors, multicolored and patterned
Hardness: 6.5 to 7 on the Mohs scale
Refractive Index: 1.530 to 1.540
Density: 2.60 to 2.64
Cleavage: None
Transparency: Translucent to opaque
Double Refraction or Birefringence: Up to 0.004
Luster: Waxy to dull
Fluorescence: Variable


Most Popular Similar or Related Gemstone Varieties and Trade Names:

Coral, banded agate, carnelian, sard, sardonyx, onyx, chrysocolla, eye agate, dendritic agate, fire agate, agate geode, moss agate and brecciated agate, amber, ammolite, and pearl are the most popular and well-known related gemstones.

Lesser-Known Similar or Related Gemstone Varieties and Trade Names:

Jet, ivory, tree agate, agate jasper, Botswana agate, blue lace agate, fossil agate, iris agate, Laguna agate, landscape agate, scenic agate, tube agate, snakeskin agate, Sweetwater agate, Mojave blue agate, thunderegg agate, Fairburn agate, Dryhead agate, Lake Superior agate and iris agate are lesser known related gemstones.

Fossil Coral Mythology, Metaphysical and Crystal Healing Properties

Since fossil coral is a combination of coral and agate, it carries many strong metaphysical beliefs. Agate is associated with the zodiacal signs Scorpio and Gemini, and agate is said to be ruled by the moon. It is a grounding stone and is supposedly good for bringing about change. Physically, agate is thought to be able to help heal pancreatic disorders and improve blood and air circulation. It is used for healing illnesses of the eyes, skin, and stomach, is even believed to enhance longevity.

Since the beginning of the 1st millennium, coral has been highly prized as a gem because it was believed to have mysterious, powerful and sacred properties. The Gauls often used coral gems as ornamentation for their helmets and weapons, believing that it was a protective stone. Ancient Egyptians also placed coral in tombs of the deceased to protect against evil spirits because they believed each coral gem actually contained divine blood.

The Romans hung branches of coral around the necks of their children to protect them from danger. In the early 20th century, the people of Italy used coral for protection from the ‘evil eye’ and many Italians used it for infertility. Among the Hopi and Zuni tribes, the ‘road of life’ is symbolized by coral, jet, abalone, and turquoise (the four elements). Although coral is not a traditional birthstone for any month, it is a non-traditional zodiac stone for Taurus. Reddish and orange colored coral stones are thought to benefit the root and spleen chakras by providing energy and warmth.

Fossil Coral Gemstone Jewelry Ideas

Agatized fossil coral is highly sought after by gem and fossil collectors around the world. It is often used in beads and other types of unique jewelry. Since fossil coral is a type of agate, it is fairly hard (7 on the Mohs scale) and very durable, rendering fossil coral perfectly suitable for all kinds of jewelry designs including daily-wear rings. Fossil coral is most frequently fashioned into pendants, but it can also make very interesting pins, brooches, necklaces, and earrings. For men’s jewelry, popular designs include cuff-links and tie tacks.

Caring for and Cleaning Fossil Coral

Fossil coral is a form of agate, which in turn is a variety of quartz. All quartz is considered quite durable compared to most other gemstones. It can be easily cleaned using warm, soapy water and a soft cloth. Be sure to rinse well to remove any soapy residue. Even though agatized fossil coral has fairly good hardness and durability, there are still a variety of other materials which can easily scratch it. Therefore, avoid wearing, mixing or storing it with other gems, whether softer or harder, in order to prevent scratches and fractures.

Always remove gems and jewelry before exercising or playing any vigorous sports. As with most gemstones, avoid the use of any harsh household chemicals (bleach, sulfuric acid and suchlike) when caring for or cleaning your fossil coral. Most forms of agate are actually quite porous, allowing them to be easily stained, meaning that they can absorb other chemicals and colors quite easily. Avoid prolonged exposure to extreme heat, as heat can permanently alter the color of your gemstone. When storing fossil coral, wrap it using a soft cloth and place it inside a fabric-lined box for added protection.

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