VI. DOUBLE REFRACTION 40 VII. ABSORPTION EFFECTS: COLOUR, DICHROISM,ETC. 53 VIII. SPECIFIC GRAVITY 63 IX. HARDNESS AND CLEAVABILITY 78 X. ELECTRICAL CHARACTERS 82 PART I—SECTION B THE TECHNOLOGY OF GEM-STONES XI. UNIT OF WEIGHT 84 XII. FASHIONING OF GEM-STONES 88 XIII. NOMENCLATURE OF PRECIOUS STONES 109 XIV. MANUFACTURED STONES 113 XV. IMITATION STONES 124 PART II—SECTION A PRECIOUS STONES XVI. DIAMOND 128 XVII. OCCURRENCE OF DIAMOND 137 XVIII. HISTORICAL DIAMONDS 157 XIX. CORUNDUM (_Sapphire_, _Ruby_) 172 XX. BERYL (_Emerald_, _Aquamarine_, _Morganite_) 184 PART II—SECTION B SEMI-PRECIOUS STONES XXI. TOPAZ 197 XXII. SPINEL (_Balas-Ruby_, _Rubicelle_) 203 XXIII. GARNET 207 (_a_) HESSONITE (_Grossular_, _Cinnamon-Stone_, _Hyacinth_, _Jacinth_) 211 (_b_) PYROPE (‘_Cape-Ruby_’) 212 (_c_) RHODOLITE 214 (_d_) ALMANDINE (_Carbuncle_) 214 (_e_) SPESSARTITE 216 (_f_) ANDRADITE (_Demantoid_, _Topazolite_, ‘_Olivine_’) 216 (_g_) UVAROVITE 218 XXIV. TOURMALINE (_Rubellite_) 219 XXV. PERIDOT 225 XXVI. ZIRCON (_Jargoon_, _Hyacinth_, _Jacinth_) 228 XXVII. CHRYSOBERYL (_Chrysolite_, _Cat’s-Eye_, _Cymophane_, _Alexandrite_) 233 XXVIII. QUARTZ (_Rock-Crystal_, _Amethyst_, _Citrine_, _Cairngorm_, _Cat’s-Eye_, _Tiger’s-Eye_) 238 XXIX. CHALCEDONY, AGATE, ETC. 246 XXX. OPAL (_White Opal_, _Black Opal_, _Fire-Opal_) 249 XXXI. FELSPAR (_Moonstone_, _Sunstone_, _Labradorite_, _Amazon-Stone_) 254