Found this mortar yesterday. First two pics of how it looked when I found it, rest are after I scrubbed it up. Where mortar is broken seems to be very glass like. Would appreciate any help with material this may be made off. Thanks.
I agree with Road Dog. What you have there is the bottom of an old hand blown bottle. I've found ones in the bottom of the river that have the same appearance as yours.
I second Road Dog. A heavily eroded bottle end blasted or washed in sand. Its pretty old anyway as such a mark in the base is from hand blowing and not a mold. My thoughts.