Rock, Mineral, and Fossil Education
Most gemstones are minerals, but a small and fascinating group comes from living organisms. Pearls grow inside mollusks, amber is ancient tree resin, jet is a type of fossilized wood, and coral is built by colonies of tiny marine animals. Because they are organic in origin, these materials fall outside the scientific definition of a mineral described in Crystals vs. Gemstones vs. Minerals, and they need different care than mineral gems.
A pearl forms when a mollusk, usually an oyster or freshwater mussel, coats an irritant or a bit of its own tissue with layers of nacre. Nacre is made of tiny plates of aragonite, a form of calcium carbonate, bound together by an organic protein called conchiolin. The overlapping plates are what give fine pearls their glow, often called luster or orient.
A traditional quick check is to rub a pearl gently against the edge of a tooth: real nacre usually feels slightly gritty, while most imitations feel smooth. At a hardness of about 2.5 to 4.5, pearls scratch easily and are damaged by acids, so they should be stored separately and wiped clean after wearing.
Amber is fossilized tree resin. Resin oozes from trees to seal wounds, and when it is buried in sediment for millions of years, its chemical components slowly link together and harden. Baltic amber, the best-known source, is generally dated at roughly 40 million years old, and amber from other deposits ranges from a few million to more than 100 million years.
Amber is famous for preserving insects, plant fragments, and occasionally small animals in remarkable detail. That preservation works differently from the mineral replacement described in How Fossils Form, because the resin seals the organism away before decay can begin.
Jet is a dense, black organic gem formed from wood that was buried and compressed under specific conditions, similar to the process that forms coal. It takes a high polish and is very lightweight. The coastal deposits around Whitby, England, are the most famous source, and Whitby jet became fashionable for mourning jewelry in Victorian Britain.
Black glass, sometimes called “French jet,” and early plastics were widely used as imitations. Genuine jet feels warm to the touch, is noticeably lighter than glass, and leaves a brown streak on an unglazed porcelain plate, a test described in Reading a Rock.
Gem coral comes from the skeletons of colonies of small marine animals called polyps. Precious red and pink coral, mostly from species of the genus Corallium, is made largely of calcium carbonate and has been carved into beads and ornaments for thousands of years. Black and golden corals are built from a protein-based material instead.
Coral harvesting is now restricted in many places because reefs and deep-sea coral beds grow very slowly, and international trade in some coral species is regulated. Older coral jewelry is common in estate collections, and dyed bone, glass, and plastic are frequent imitations.
Organic gems share a few vulnerabilities that make them more delicate than most mineral gems:
For general advice on storing a mixed collection, see Caring for a Mineral Collection.
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