I am not an expert by any means, but I think you have a piece of pirate glass. Most common color when held to the light is an olive green which you have. The piece looks thick too. Google pirates glass to get more info. I think I found one last year which on the sand looked black, but when held up to the light was amber and has what looks like a couple of lions marching across. What a good find you have! Also, I love your other pieces!!!! Keep them!