Fossilized organisms

Chemistry: CaCO3

Crystallography: Amorphous

Refractive index: 1.567 - 1.590

Hardness: 4.5-5

Specific gravity: Usually 2.70

Cleavage: No

Heat sensitive: Yes

Wearability: Poor (very brittle)

Special care instructions: None

Enhancements: Frequent, Impregnation (crack filling)

Most people who have an interest in gemstones or nature have seen petrified wood. 

Fewer are aware of the many other types of fossilized organisms that can be fashioned into beautiful gems. 

Sometimes the whole organism is preserved as with Turritella snails, Starburst Algae, or Sea Lilies. 

More often, though, various parts or products are all that are preserved, such as bone, shell, eggshell, footprints, tunnels, or even dung.

The preservation occurs most often as the organic materials are slowly replaced with mineral solutions, such as silica, which hardens and takes the original shape. 

In other cases, mud in which the structure lies is compacted and compressed over time into shale that now holds the fossil.

Particularly beautiful are the ammolites derived from the name of a snail-like organism (ammonite) that lived around 70 million years ago. 

Ammolites are the result of the replacement of the thin layers of the shell of an ammonite with mineral layers, which then create iridescence.

This gem grade material comes from Southern Alberta, Canada, and nowhere else in the world. 

Gem material like this is found buried under a forest of lodgepole pine and trembling aspen. 

The ammonite shell's outer layer consists of aragonite and is used to make ammolite gems. 

This layer is usually very thin (up to 1mm thick) and is fragile. 

Ammolite gems are typically fabricated into triplets (same principle as for the opal triplets) with quartz (hardness 7), synthetic spinel (hardness 8, less brittle than quartz), or synthetic corundum (hardness 9, best quality) top layers and a black Onix backing.

There are no accepted price ranges for fossilized organisms, other than the general considerations that apply to all gems. 

Common items, like shark's teeth, are generally inexpensive, while rarer items like fossilized ivory or dinosaur eggshell command higher prices. 

In any given case, specimens with the best patterns or most complete preservation of the organism in question are most valued.