It is a "slab built" vase. I call the style "hippy pottery". it was popular in high schools, colleges and art fairs in the 1970s. It is unusual that it isn't signed. It may have been hand made commercially in Mexico, Indonesia, India, etc. It looks like an indigenous clay, rather than commercial clay.
A blob of clay is rolled out with a rolling pin and the sides of the vase cut from it then pressed together. In this case a pine branch was first set down and then the clay rolled over it to make the impression, the same branch appears to have been flipped over for the other side. The clay is quite thick so the branch impression wouldn't compromise the integrity of the wall. That's why it's so heavy.
