Many gem dealers have been deterred from beginning a study of gems because of the seeming difficulties in connection with the scientific determination of the different varieties of stones. Now science is nothing but boiled-down common sense, and a bold front will soon convince one that most of the difficulties are more apparent than real. Such minor difficulties as exist will be approached in such a manner that a little effort will overcome them. For those who are willing to do more work, this book will suggest definite portions of particular books, which are easily available, for reference reading and study--but the lessons themselves will attempt to teach the essential things in as simple a manner as is possible. Perhaps the first essential for the gem merchant is to be able surely to distinguish the various stones from one another and from synthetic and imitation stones. That such ability is much needed will be clear to anyone who in casting a backward glance over his experience recalls the many serious mistakes that have come to his knowledge. Many more have doubtless occurred without detection. Several times recently the author has come across cases where large dealers have been mistaken in their determination of colored stones, particularly emeralds. Only the other day a ring was brought to me that had been bought for a genuine emerald ring after the buyer had taken it to one of the dealers in his city and had paid for an examination of it, which had resulted in its being declared genuine. On examining the stone with a lens of only moderate power, several round air bubbles were noted in it, and on barely touching it with a file it was easily scratched. The material was green glass. Now, what was said about the dealer who sold it and the one who appraised it may be imagined. The long chain of adverse influence which will be put in action against those dealers, even though the one who sold the stone makes good the loss, is something that can be ill afforded by any dealer, and all this might have been avoided by even a rudimentary knowledge of the means of distinguishing precious stones. The dealer was doubtless honest, but, through carelessness or ignorance, was himself deceived. Our first few lessons will therefore be concerned chiefly with learning the best means of telling the different stones from one another. CONTENTS PAGE PREFACE iii LESSON I.--HOW STONES ARE DISTINGUISHED FROM ONE ANOTHER 1 II.--REFRACTION 4 III.--DOUBLE REFRACTION 8 IV.--ABSORPTION AND DICHROISM 15 V.--SPECIFIC GRAVITY 23 VI.--SPECIFIC GRAVITY DETERMINATIONS 31 VII.--LUSTER AND OTHER REFLECTION EFFECTS 38