Very dark stone--can it be turquoise?

I found this ring through an online auction. I think it’s really special in an understated way, so I passed it on to my son-in-law, who has similar characteristics.

I assume that the maker, Holgate, was Eugene Holgate, a Navajo artist. Please correct me if I’m wrong.

The central stone is very dark but has great depth. Is it turquoise? Or what could it be?
PS-photos are from the auction


It is a really interesting ring. Maybe it is something fossilized like dinosaur bone? Gem Quality 6.59 grams Agatized Dinosaur Gembone Fossil Cabochon


Jason, thanks for that information. It makes the ring even more fascinating.

That is a beautiful ring. I am stumped by the stone, but the overall workmanship is awesome!