For decades, GEMS has been a standard reference book for gemologists worldwide. The third, fourth, and fifth editions all had terrific indexes where most everything, common and uncommon, was listed and referenced.
The sixth edition, edited by Michael O’Donoghue, has many excellent additions, especially in the organics and pearls sections, but it doesn’t list basic historical references, such as Russia for demantoid garnet. And demantoid is nowhere in the index. The index entry for andradite garnet advises the reader to “see Chapter 11,” with no page number.
The book is still worth buying for the new material. Just don’t expect it to be easy to navigate. And keep your fifth edition handy as a backup.