chickweed on Nostr: Opals form in cavities within rocks. If a cavity has formed because a shell was ...

Opals form in cavities within rocks. If a cavity has formed because a shell was buried in sand or clay that later became rock, and conditions are right for opal formation, then it forms a fossil replica of the original object.

https://www.geologypage.com/2017/04/opalised-fossils-form.html

#geology

photo: https://www.pinterest.com/pin/535506211945205379/