Emerald, unlike some green stones, retains its purity of colour in artificial light; in fact, to quote the words of Pliny, “For neither sun nor shade, nor yet the light of candle, causeth to change and lose their lustre.” Many are the superstitions that have been attached to it. Thus it was supposed to be good for the eyes, and as Pliny says, “Besides, there is not a gem or precious stone that so fully possesseth the eye, and yet never contenteth it with satiety. Nay, if the sight hath been wearied and dimmed by intentive poring upon anything else, the beholding of this stone doth refresh and restore it again.” The idea that it was fatal to the eyesight of serpents appears in Moore’s lines— “Blinded like serpents when they gaze Upon the emerald’s virgin blaze.” The crystals occur attached to the limestone, and are therefore never found doubly terminated. The crystal form is very simple, merely a hexagonal prism with a flat face at the one end at right angles to it. They are invariably flawed, so much so that a flawless emerald has passed into proverb as unattainable perfection. The largest single crystal which is known to exist at the present day is in the possession of the Duke of Devonshire (Fig. 71). In section it is nearly a regular hexagon, about 2 inches (51 mm.) in diameter from side to side, and the length is about the same; its weight is 276·79 grams (9¾ oz. Av., or 1347 carats). It is of good colour, but badly flawed. It was given to the Duke of Devonshire by Dom Pedro of Brazil, and was exhibited at the Great Exhibition of 1851. A fine, though much smaller crystal, but of even better colour, which weighs 32·2 grams (156½ carats), and measures 1⅛ inch (28 mm.) in its widest cross-diameter, and about the same in length, was acquired with the Allan-Greg collection by the British Museum, and is exhibited in the Mineral Gallery of the British Museum (Natural History). The finest cut emerald is said to be one weighing 30 carats, which belongs to the Czar of Russia. A small, but perfect and flawless, faceted emerald, which is set in a gold hoop, is also in the British Museum (Natural History). It is shown, without the setting, about actual size, on Plate I, Fig. 5. [Illustration: FIG. 71.—Duke of Devonshire’s Emerald. (Natural size.)]