There have been several posts recently where people have asked for help identifying unknown materials.  Some of the responses have asked for the original poster to measure the specific gravity of the specimen (mine included).  I realized that this is not something everyone knows how to do, so I wrote up the attached document to explain it.  Sorry it's a bit long, but it includes a discussion of what specific gravity is, how it can be useful, some limitations with using it, and a couple of ways t