The title defines the emphasis. And the lay reader may be put off by such chapters as ""The Refractive Index and its Measurement."" But it is passably readable and offers a great deal of information about the origin and composition of gems. The first section gives a brief chronology of the history and separate attributes accorded to them. There are interesting sidelights about diamond cutting, mining and the cultured pearl industry. There is a complete analysis of the various effects of light upon the different types of jewel, a chapter devoted to each specimen and instructions for private testing.