What is Fossil Coral?

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FOSSIL CORAL, from Construction Material, Health Supplements, and now with
discovery of uniquely preserved Indonesian varieties, the ornate use in Jewelry
Joel Ivey 2008


    It is important that people understand "fossil coral" is a natural stone formed from ancient corals. It should not be mistaken for protected and endangered coral reef from the modern oceans of today.  Of the varieties of fossilized corals found throughout the geologic record,  exquisitely detailed specimens from the mountains of Indonesia are most unique and beautiful. Corals have been growing in the oceans around the world for almost 500 million years. The corals thrived in warm shallow marine waters and over time were buried in sediments as the land surface was juggled by crustal plate movements or the oceans rose and fell.   Temperature and pressure from compaction during burial, resulted in those deposits, in time, becoming rock and part of the present day geological record. Decorative coraline limestone and marble are popular building materials and have been used for centuries.For many years, ancient coral deposits have been mined for their high quality calcium, potassium, magnesium and sodium content which is used in health supplements.  Fossil coral has also been used in fertilizers.  Fossil coral has even been used in filters to remove impurities such as chlorine and formaldehyde from water.  Ornate coral pieces are also sought by fossil collectors or used in beads and other jewelry.  Silurian age (390 million year old) horn corals from Utah (FIGURE 1)   are prized for fine definition and brilliant red color.  Geodes formed by partial replacement or casts of coral heads by low temperature chalcedonic or opaline silica are found in Florida.  Florida has even declared fossil coral to be the "Florida State Rock".  The shapes and colors of these specimens from the Tampa Bay region, in north Florida and southern Georgia are unique.  They are associated with Tertiary rock formations of 20 to 25 million years in age. (FIGURE 2) 

 Fig 1.  Fig 2.


    Silicification of Paleozoic Era corals such as the Devonian age (416 to 359 million years ago) "Petosky Stone" of the Lake Michigan region (FIGURE 3)   or on Prince of Wales Island, Alaska, are attributed to the large amount of siliceous sponges which flourished in the ancient warmer shallow waters.  The sponges represent a likely source of mobile silica in the sedimentary pile.   As the sponges decomposed, the silica was freed to replace the carbonate rich coral skeletons.   Specimens are dominantly gray colored and exhibit a hexagonal pattern typical of primitive coral species.


 Fig 3.


    Let's begin with a brief review of the theory of plate tectonics. New ocean crust is created by upwelling basaltic magma along the mid-ocean ridges.  Ocean plates move away from mid-ocean spreading centers and subduct under the continental plates.  Subducting ocean plate material re-melts as it reaches depths under high temperature and pressure and becomes molten magma which seeps back up to the surface and erupts thru volcanic vents.  This occurrence is most evident along the Pacific Rim, what is commonly known as the ring of fire, an area depicted by high incidence of volcanic activity.   Indonesia is located on convergent plate boundaries.  Subduction of the northeast moving Indian Ocean plate under the western islands of the Indonesian archipelago has resulted in a long history of volcanic activity (FIGURE 4).


Fig 4.


    Extensive eruptions of volcanic debris have repeatedly buried forests and nearby reefs offshore as well.  During burial, the decay of the volcanic ash releases mobile silica and other elements which saturate formation waters and in some situations reacts with the carbonate of the buried coralline reefs or other organic matter such as wood from buried forests.    The volcanic islands of Indonesia are host to numerous deposits of fossil coral.  The Barisan Mountain Range, along the western boundary of the Island of Sumatra, hosts a rare and ornate suite of fossils.  Miocene age (approx 20 million year old) coralline reefs and near shore forests were preserved by burial in volcanic sediments rich in minerals including iron, manganese and silica.  Thru a combination of processes the full pattern and character of the original coral life forms have been preserved in great detail. 


    "Permineralization" is the process of filling pore space in and around the remnant hard coral skeleton with minerals deposited from solutions trapped or migrating thru the sedimentary pile as it is compressed into rock.  "Replacement" is the process whereby the original coral skeleton is replaced molecule by molecule with a mineral or minerals from a solution.  For example, calcium carbonate (CaCO3) from the hard structure of the coral is replaced by silica (SiO2) from trapped or migrating solutions as rock was being formed. This dual process preservation can occur with different accessory mineral concentrations and result in maintaining the contrast between the original soft tissues and the skeletal remains of the corals as different minerals impart different colors to the stone. The geochemical and geological conditions under which these processes take place are typically slightly acidic, low temperature and low pressure.   The resultant deposition of replacement product is a micro or crypto crystalline quartz commonly termed "agate".  In Indonesia, entire coral heads (FIGURE 5)    are often completely preserved and appear just as they did 20 million years ago although their density is much changed by replacement and infill with silica, iron, manganese and other minerals.  There are fern corals, brain corals, hex corals, honeycomb corals and many more.Specimen coral heads are found in many stages of preservation.  Some are mere casts with thin replacement of the outer skin and a pumaceous, leached or partial replacement of the inner core.  Other coral heads are completely preserved by the dual process and can make beautiful polished slab specimens (FIGURE 6).    The silica ranges from light weight amorphous opaline to dense cryptocrystalline chalcedony.  Re-crystalization is often found on fractures resulting in fine glittering coatings of quartz druze.  Rarely, entire specimens are coated in fine crystals.   The fossilized coral heads come in all sizes and can weigh from 100grams to 100 kilograms.I found my first specimen while conducting a regional geological mapping program in South Sumatra in 1988.  The smooth dark green, water worn cobble weighed about 2kg.  It was solidly agatized and the honeycomb coral pattern was well defined.  Sadly, the driver later took it to a machine shop and ground the center out to make an ashtray.  Such is Indonesia.

 Fig 5. 

 Fig 6.


    In 2001, I returned to the area and found the local villagers had begun a brisk business of digging the coral and smashing it up with hammers then fashioning the chips (FIGURE 7)  into ornate cabochons and beads.  I helped them find export markets, build some rock saws and from the workbenches of friends in the USA and Thailand, brought in some popular shapes to cut.  This caught on and there is now a flourishing local lapidary industry in Indonesia  (FIGURE 8) and their mainstay is agatized fossil coral.  We continued to survey the region and soon we had identified other deposits along the Barison Mountain Range.  These were assorted characters and degrees of preservation.  A great many coral species are preserved in three Sumatran Provinces and there is an occurrence in West Java and another in East Java as well.  The assortment of accessory minerals introduced with the silica, vary greatly in color, from white to red and yellow, purple and pink (FIGURE 9). In several areas, we found proximal agatized palms and other fossilized woods which may have been preserved by a single volcanic event.


 Fig 7.  Fig 8.  Fig 9.


    Over the last 10 years, the unique ornate character of Indonesian agatized fossil coral has become recognized by rock hounds, fossil collectors, jewelers and lapidary enthusiasts around the World.  If you GOOGLE just about any combination of words related to fossil coral or jewelry & coral, you will be directed to a myriad of websites with rough or polished Indonesian Agatized Fossil Coral materials (FIGURES 10 & 11)  .  Indonesian Fossil Coral products are found at Rock & Mineral and Gem & Jewelry Shows around the world.



    Agate is a common term for a naturally occurring variety of chalcedony or microcrystalline quartz.  Typical colors are earthy tans, browns, grays, yellows, oranges, reds, pinks and purples.Zodiac associations of agate are Gemini and Virgo.  Astrologically agate is associated with the Moon.Agate is said to enhance one's perceptiveness and concentration; and to stimulate analytical capacity.  Agate is thought to provide balanced yin/yang energies and to promote harmony as well as emotional and spiritual strength and self-confidence.  In romance, agate is said to represent marital and romantic fidelity.Used in contact with the position of the 3rd Chakra (Solar Plexus), agate eases stress and anxiety by grounding emotions.Physically, agate is thought to promote health of the teeth and gums and help heal disorders of the pancreas and improve circulation.  It was a traditional healer of eye, skin and stomach ailments and was said to enhance longevity.Agate is also believed to prevent insomnia and ensure pleasant dreams.



    The physical toughness, visual softness, multitude of colors and graphic character exhibited by the agate which has replaced and preserved corals of Indonesia, provide a vast range of associated correlatives with the agates of the Metaphysical world.The fact that forces of nature have created these stones in the image of once living creatures, assures that, although all have similar physical properties, every piece will appear unique unto its viewer and no two pieces will be exactly alike. All these features add to the appeal of Indonesian Agatized Fossil Coral and its ever increasing popularity around the world today by curators, collectors, jeweler, hobbyists and rock hounds


Mygemgarden thanks Mr. Ivey for his comments and information on how the Fossil Coral industy was shaped in Indonesia. The top fossil coral cabochons are quite rare, and can be found in the market for $100 to $300 and more. Even at top gem shows, you won't find much of the top fossil coral as collectors buy it up before it even hits the market. Much of it remains in Indonesia, where the residents are very interested in their own rock artifacts. At the current time, the government does not allow the fossil coral rough material to be exported, making the material even more rare. A lot of substandard material was once shipped to China, and you will see a lot of that on the market in various products. The actual material from the rough that could make gem grade cabochons is less than 5%, maybe even 1%. Some patterns are very rare. The finished cabs that are exported from Indonesia, often hand carried by merchants, is usually hand polished, often with banana leaves.


  • If you read and found the above material interesting and useful, please leave us a positive comment by placing a check in the box. We plan more guides of  other rare Indonesian material right from the very people that spent years there shaping the market.


Bob Sills, Mygemgarden (Detroit, Mi.)



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