Organic Gemstones: This colorful collage features some of the organic gems of this article. Starting at the top and moving to the left are: amber, ammolite, fossil coral, petrified wood, fossil stromatolite, mother of pearl, fossil sand dollar, turritella agate, freshwater cultured pearls, fossil dinosaur bone, fossil crinoidal limestone, and red coral. See below for more.
What are Organic Gems?
Although most gemstones are mineral materials, a number of organic materials are also considered to be gemstones. The most common of these are pearl, bone, amber, coral, jet, and ivory. These are materials, produced by organisms, that have been cut into gems and other ornamental objects.
There are also organic materials that have been mineralized (replaced and infilled by chalcedony, opal, calcite, aragonite, pyrite, or other mineral material). Although the material itself is not organic, it does preserve an organic structure. Examples include petrified wood, fossil coral, dinosaur bone, and other fossilized organisms or parts of organisms.
"Turritella Agate" is the name given to a brownish gem material that contains spectacular fossil snail shells entombed in a semitransparent agate. Although millions of people have called this material "Turritella" for several decades, the name is actually incorrect. It was mistakenly named after a genus of fossil snails that are very similar to the shells in the agate. The proper name of the snails is "Elimia tenera," a member of the Pleuroceridae family. Perhaps a more accurate (although less elegant) name for the material would be "Elimia Agate."
About 50 million years ago, the spiral-shaped shells accumulated in the sediments of a shallow inland sea in an area that we now know as the state of Wyoming. A few lenses of snail-bearing sediment, in what is today known as the Green River Formation, were then agatized by the deposition of fine-grained silica (chalcedony - also known as agate) into the cavities of the shells and the voids between them. If the sediment was completely agatized, it has potential lapidary (gem cutting) potential. To learn more about the Turritella - Elimia naming error, visit the Paleontological Research Institution - people who know what they are talking about when it comes to fossils.
Mary Ellen Jasper is a rock found in Minnesota that consists of red jasper and silver hematite. The jasper is a fossil stromatolite, a layered structure built up by sediment-trapping algae. The algae that produced the stromatolite structures in Mary Ellen lived on Earth about two billion years ago - long before land plants.
Author: Hobart M. King, Ph.D.