The word carat comes from the Carob Mediterranean tree whose seed was used for centuries as the standard of weighing precious gems. Each carob seed weighs approximately one-fifth of a gram. A carat is one-fifth of a gram.
Jewelry & Diamond Industry Dictionary of Terminology
Specific Gravity (SG)
Density, measured as the weight of the material compared with that of an equal volume of water.
Sources for these definitions are Cally Hall, GIA.edu, Google.com, Meriam-Webster.com, and Wikipedia.org.