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Biogenic precious gems are gemstones of biological origin, created through natural biological processes rather than geologic processes. Depending on the stone in question, some biogenic precious gems are totally biogenic, while others involve a mixing of natural and geological processes: gem-quality fossils, for example, are biogenic but also mineral in nature. The value of biogenic precious gems varies considerably, depending on the gem and the quality.
Some well-known examples of biogenic precious gems include: pearl, coral, mother of pearl, and ivory. These precious materials are formed by a variety of organisms for a variety of purposes. Mother of pearl, for example, forms as a result of deposition of layers of material called nacre, used to expand the shells of some marine mollusks, while ivory is a type of tooth present in elephants and rhinoceroses, among others.
Other biogenic precious gems like amber and ammolite consist of fossilized remains of natural materials. In the case of amber, the material is sap, which may also trap insects and other inclusions which make the resulting gem more biologically interesting. Ammolite is made from the remains of fossilized ammonites, marine mollusks which swam the oceans millions of years ago. Some people refer to fossilized gems as minerals rather than biogenic gems, since geologic processes are involved in their formation.
The appearance of biogenic precious gems varies widely, as you can see from the above examples. Some biogenic precious gems are translucent with flecks and inclusions, like amber, while others are solid, as is the case with ivory. Colors may vary significantly, and so does hardness. Some biogenic precious gems are extremely fragile, and they must be handled with care and set carefully to ensure that they will not crack, cloud, or break, while others are sturdy enough to be used as pool balls.
When evaluating the quality of biogenic precious gems, people should look at things like clarity, cut, and color, just like with diamonds, and they should also check for obvious cracks, flaking, or other signs of damage. In the case of set gems, the setting should be firm and stable, and in the case of fragile gems, it should be protected or the gems should be capped in hard crystal to reduce the risk of loss or damage. There are also some ethical issues involved in biogenic gems, with ivory for example being particularly ethically challenging due to widespread poaching of elephants for their ivory. There are no certification programs to confirm the origins of biogenic precious gems for concerned consumers, which can make it difficult to buy ethically, and some people avoid controversial gems like coral and ivory altogether for this reason.