Many fossils are hard and colorful enough to cut and polish for jewelry. The best known are ammolite from Canada, whole ammonite shells, petrified wood, agatized coral and agatized dinosaur bone. In most of them, minerals such as silica have replaced or filled the original tissue, while ammolite keeps the original shell material. Softer or porous fossils are often stabilized with resin before they are made into jewelry.
How fossils become gem material
A fossil forms when the remains of a living thing are buried and preserved in rock. In some fossils the original material survives with little change. In others, mineral-rich groundwater seeps through the buried remains over long periods. Minerals fill the pores and cells, a process called permineralization, and sometimes replace the original material entirely. When the replacing mineral is fine-grained silica, the fossil turns into agate, chalcedony or jasper and becomes hard enough for daily wear.
Most of these fossils form in sedimentary basins, and the page on sedimentary rocks and gemstones describes those settings. Because their origin is biological, fossils are often grouped with the organic gems, even when their present makeup is mineral.
Ammolite
Ammolite is the iridescent shell of fossil ammonites from the Bearpaw Formation in southern Alberta, Canada. The ammonites lived in an inland sea in the Late Cretaceous, roughly 70 to 75 million years ago. Two species, Placenticeras meeki and Placenticeras intercalare, provide most of the gem material.
The shell is made of aragonite, the same form of calcium carbonate found in pearl nacre. Thin, stacked layers break up light into strong colors through interference, much like the play of color in opal. Red and green are the most common colors, while blue and purple are rarer. The World Jewellery Confederation, CIBJO, accepted ammolite as a gemstone in 1981.
Ammolite layers are very thin and fragile, often less than a millimeter thick, and the material often cracks into a pattern described as dragon skin or stained glass. Most ammolite jewelry is made as doublets or triplets. A doublet bonds the thin ammolite layer to a dark backing such as shale. A triplet adds a clear protective cap of quartz or synthetic spinel. Solid ammolite exists but is less common and more easily damaged.
Whole ammonites
Ammonites are extinct relatives of squid and octopus with coiled, chambered shells. They lived for hundreds of millions of years before dying out at the end of the Cretaceous. Fossils from Madagascar and Morocco are often cut in half and polished to show the internal chambers, which may be filled with calcite crystals or agate.
Small polished ammonites and slices appear in pendants and earrings. Some Madagascar ammonites keep an iridescent shell layer, though usually less intense than Alberta ammolite.
Petrified wood
Petrified wood forms when silica replaces the tissues of buried trees, often preserving tree rings and cell structure. Iron and manganese give it reds, yellows, browns and blacks. It is hard, about 6.5 to 7 on the Mohs scale, and polishes to a glassy luster. Opalized wood from Australia and Nevada contains opal instead of chalcedony. The page on wood and petrified wood in jewelry covers it in full, along with jet and bog oak.
Fossil coral
Ancient coral colonies can be replaced by silica, creating agatized coral. Florida produces agatized coral from Tampa Bay and other sites, and the state adopted it as its state stone in 1979. The flower-like pattern of the coral polyps shows clearly on polished surfaces.
Petoskey stones from Michigan are fossil colonial coral, Hexagonaria, from the Devonian period. They show a pattern of six-sided cells and were named Michigan's state stone in 1965. Petoskey stones are made of calcite and are soft, so they suit pendants and display pieces better than rings. Fossil coral is a different material from modern precious red coral.
Dinosaur bone
Agatized dinosaur bone, sometimes sold as gem bone, comes mainly from the Colorado Plateau in Utah, Colorado and Arizona. Silica and other minerals filled the porous inner bone, preserving its cell structure. Polished pieces show a honeycomb of small cells in red, yellow, gray or blue. Most gem-grade dinosaur bone cannot be traced to a particular species.
Collecting vertebrate fossils on US federal land requires a permit, and fossils sold legally come from private land. Rules differ in other countries, and some ban the export of fossils.
Fossil jasper and other materials
Several patterned stones sold as jasper contain fossils. Turritella agate from Wyoming is packed with the spiral shells of small freshwater snails. Despite its name, the snails are a genus called Elimia, not Turritella. Stromatolite jasper contains layered structures made by ancient microbial mats. Fossil palm wood shows rod-like fibers in cross-section. These stones are cut into cabochons and beads like other jasper.
Fossil ivory from mammoths is another fossil material used in carving and jewelry. Its trade is covered on the page about ivory in jewelry.
Stabilization and treatments
Many fossils are porous, fractured or soft. To make them workable, cutters often impregnate them with resin or epoxy under vacuum, a process called stabilization. It fills cracks and pores and makes the stone tougher. Ammolite and dinosaur bone are commonly stabilized, and some fossils are also dyed or backed to improve color.
Stabilization is a treatment and should be disclosed. The page on gemstone treatments explains common treatments and how they affect care.
Care
Care depends on the material. Silica-based fossils such as agatized coral and petrified wood are fairly durable. Ammolite, Petoskey stones and ammonite shell are soft and sensitive to acids, and doublets and triplets can separate if soaked or heated. Clean fossil jewelry with a soft cloth and mild soapy water, and keep it away from ultrasonic and steam cleaners. The page on cleaning jewelry safely has more detail by material.
Frequently asked questions
Is ammolite a real gemstone?
Yes. Ammolite is the iridescent fossil shell of ammonites from Alberta, Canada, and CIBJO accepted it as a gemstone in 1981.
Why is most ammolite sold as a triplet?
The colored layer is thin and fragile. A backing and a clear cap of quartz or spinel protect it and make it practical to wear.
How hard is petrified wood?
Most petrified wood is replaced by silica and rates about 6.5 to 7 on the Mohs scale, similar to agate and quartz.
Is dinosaur bone jewelry legal?
Dinosaur bone sold in the United States generally comes from private land, where collecting is legal. Collecting on federal land needs a permit, and some countries ban fossil export.