What is it called?...

[PullTabProphet]

When silver or gold coins from say, a lost treasure are found at the bottom of the sea and they are encased in that "concrete-like" crud...what is the proper word for that? It's on the tip of my tongue but I can't remember lol!
Thanks for any smart answers to my dumb question!

PullTabProphet

 

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[villagenut]

Concretion....

 

[PullTabProphet]

Concretion....

That's it. Thanks!

 

[Red-Coat]

Concretion would be appropriate but, in marine environments, the more usual archaeological term is "encrustation":

Conservation of Marine Encrustations: What Composites of Marine Sediments Can Tell Us About Shipwrecks, Artifacts, and Oceanic Environments | Real Archaeology

 

[Gare]

GREAT Job VILLAGENUT !!!