Webster, Robert. Gems. ,
Butterworth and Co Ltd., 1962.
First published in 1962 and currently up to it's 6th edition, this book is a cornerstone of gemology. Robert Webster set a standard with the original work and each editor of subsequent editions has tried to lift a good thing to even higher standards. The book holds nearly 600 pages of extensive description of most gem species followed by an in depth account on gem identification.
Acquiring a copy can be a bit of a challenge, if you run into an affordable copy I suggest you buy it without hesitating.