The diagram on Plate II and most of the crystal drawings were made by me. The remaining drawings are the work of Mr. H. H. Penton. He likewise prepared the coloured drawings of cut stones which appear on the three coloured plates, his models, with two exceptions, being selected from the cut specimens in the Mineral Collection of the British Museum by permission of the Trustees. Unfortunately, the difficulties that still beset the reproduction of pictures in colour have prevented full justice being done to the faithfulness of his brush. I highly appreciate the interest he took in the work, and the care and skill with which it was executed. My thanks are due to the De Beers Consolidated Mines Co. Ltd., and to Sir Henry A. Miers, F.R.S., Principal of the University of London, for the illustrations of the Kimberley and Wesselton diamond mines, and of the methods and apparatus employed in breaking up and concentrating the blue ground; to Messrs. I. J. Asscher & Co. for the use of the photograph of the Cullinan diamond; to Mr. J. H. Steward for the loan of the block of the refractometer; and to Mr. H. W. Atkinson for the illustration of the diamond-sorting machine. My colleague, Mr. W. Campbell Smith, B.A., has most kindly read the proof-sheets, and has been of great assistance in many ways. I hope that, thanks to his invaluable help, the errors in the book which may have escaped notice will prove few in number and unimportant in character. To Mr. Edward Hopkins I owe an especial debt of gratitude for his cheerful readiness to assist me in any way in his power. He read both the manuscript and the proof-sheets, and the information with regard to the commercial and practical side of the subject was very largely supplied by him. He also placed at my service a large number of photographs, some of which—for instance, those illustrating the cutting of stones—he had specially taken for me, and he procured for me the jewellery designs shown on Plates IV and V. If this book be found by those engaged in the jewellery trade helpful in their everyday work, and if it wakens in readers generally an appreciation of the variety of beautiful minerals suitable for gems, and an interest in the wondrous qualities of crystallized substances, I shall be more than satisfied. G. F. H. S. WANDSWORTH COMMON, S.W. CONTENTS CHAP. PAGE I. INTRODUCTION 1 PART I—SECTION A THE CHARACTERS OF GEM-STONES II. CRYSTALLINE FORM 6 III. REFLECTION, REFRACTION, AND DISPERSION 14 IV. MEASUREMENT OF REFRACTIVE INDICES 21 V. LUSTRE AND SHEEN 37