The ever great demand and the essentially restricted supply have forced the cost of emeralds of good quality to a height that puts large stones beyond the reach of all but a privileged few who have purses deep enough. The rate per carat may be anything from £15 upwards, depending upon the purity of the colour and the freedom from flaws, but it increases very rapidly with the size, since flawless stones of more than 4 carats or so in weight are among the rarest of jewels; a perfect emerald of 4 carats may easily fetch £1600 to £2000. It seems anomalous to say that it has never been easier to procure fine stones than during recent years, but the reason is that the high prices prevailing have tempted owners of old jewellery to realize their emeralds. On the other hand, pale emeralds are worth only a nominal sum. The other varieties of beryl are much less rare, and, since they usually attain to more considerable, and sometimes even colossal, size, far larger stones are obtainable. An aquamarine, particularly of good deep blue-green colour, is a stone of great beauty, and it possesses the merit of preserving its purity of tint in artificial light. It is a favourite stone for pendants, brooches, and bracelets, and all purposes for which a large blue or green stone is desired. The varying tints are said to be due to the presence of iron in different percentages, and possibly in different states of oxidation. Unlike emerald, the other varieties are by no means so easily recognized by their colour. Blue aquamarines may easily be mistaken for topaz, or vice versa, and the yellow beryl closely resembles other yellow stones, such as quartz, topaz, or tourmaline. Stones which are colourless or only slightly tinted command little more than the price of cutting, but the price of blue-green stones rapidly advances with increasing depth of tint up to £2 a carat. The enormous cut aquamarine which is exhibited in the Mineral Gallery of the British Museum (Natural History), affords some idea of the great size such stones reach; a beautiful sea-green in colour, it weighs 179·5 grams (875 carats), and is table-cut with an oval contour.