Each paragraph should end with at least one reference, as long as it is not trivial and common knowledge. References in the middle of a paragraph are usually not valid for the sentences afterwards in the same paragraph.
The chemical composition is only given as 70-75% (probably percent by weight?) SiO2 plus MgO, Fe3O4, and water. This should be refined.
The natural weathering process of obsidian is only very shortly described (transformation to perlite) and should be expanded. It is used in archeology for estimating the age of obsidian artifacts such as spear heads and knifes.
