That a gem-stone should be a delight to the eye is a truism that need not be laboured; for such is its whole _raison d’être_. The members of the Mineral Kingdom that find service in jewellery may be divided into three groups, according as they are transparent, translucent, or opaque. Of these the first, which is by far the largest and the most important, may itself be further sub-divided into two sections: stones which are devoid of colour, and stones which are tinted. Among the former, diamond reigns supreme, since it alone possesses that marvellous ‘fire,’ oscillating with every movement from heavenly blue to glowing red, which is so highly esteemed and so much besought. Other stones, such as ‘fired’ zircon, white sapphire, white topaz, and rock-crystal, may dazzle with brilliancy of light reflected from the surface or emitted from the interior, but none of them, like diamond, glow with mysterious gleams. No hint of colour, save perhaps a trace of the blue of steel, can be tolerated in stones of this category; above all is a touch of the jaundice hue of yellow abhorred. It taxes all the skill of the lapidary to assure that the disposition of the facets be such as to reveal the full splendour of the stone. A coloured stone, on the other hand, depends for its attractiveness more upon its intrinsic hue than upon the manner of its cutting. The tint must not be too light or too dark in shade: a stone that has barely any colour has little interest, and one which is too dark appears almost opaque and black. The lapidary can to some extent remedy these defects by cutting the former deep and the latter shallow. In certain curious stones—for instance tourmaline—the transparency, and in others—such as ruby, sapphire, and one of the recent additions to the gem world, kunzite—the colour, varies considerably in different directions. The colours that are most admired—the fiery red of ruby, the royal blue of sapphire, the verdant green of emerald, and the golden yellow of topaz—are pure tints, and the absorption spectra corresponding to them are on the whole continuous and often restricted. They therefore retain the purity of their colour even in artificial light, though certain sapphires transmit a relatively larger amount of red, and consequently turn purple at night. Of the small group of translucent stones which pass light, but are not clear enough to be seen through, the most important is opal. It and certain others of the group owe their merit to the same optical effect as that characterizing soap-bubbles, tarnished steel, and so forth, and not to any intrinsic coloration. Another set of stones—moonstone and the star-stones—reflect light from the interior more or less regularly, but not in such a way as to produce a play of colour. The last group, which comprises opaque stones, has a single representative among ordinary gem-stones, namely, turquoise. In this case light is scattered and reflected from layers immediately