Gemstone ring set in sterling silver

Unclear which type of rock / gem this beauty is, as I'd never seen it before or since. I don't wear it though and it's too unique just to sit. As a slight pockmark on one of the striations but is not noticeable. See last picture. Inside of ring reads 92.5 meaning it's 92.5% silver. I am a ring size 4 and this only stays on my pointer finger. It is a size 5.5-6