While the treatment has been kept as simple as possible, technical expressions, where necessary, have not been avoided, but their meanings have been explained, and it is hoped that their use will not prove stumbling-blocks to the novice. Unfamiliar words of this kind often give a forbidding air to a new subject, but they are used merely to avoid circumlocution, and not, like the incantations of a wizard, to veil the difficulties in still deeper gloom. For actual practical work the pages on the refractometer and its use and the method of heavy liquids for the determination of specific gravities, and the tables of physical constants at the end of the book, with occasional reference, in case of doubt, to the descriptions of the several species alone are required; other methods—such as the prismatic mode of measuring refractive indices, or the hydrostatic way of finding specific gravities—which find a place in the ordinary curriculum of a physics course are described in their special application to gem-stones, but they are not so suitable for workshop practice. Since the scope of the book is confined mainly to the stones as they appear on the market, little has been said about their geological occurrence; the case of diamond, however, is of exceptional interest and has been more fully treated. The weights stated for the historical diamonds are those usually published, and are probably in many instances far from correct, but they serve to give an idea of the sizes of the stones; the English carat is the unit used, and the numbers must be increased by about 2½ per cent. if the weights be expressed in metric carats. The prices quoted for the various species must only be regarded as approximate, since they may change from year to year, or even day to day, according to the state of trade and the whim of fashion.
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.