In recent years, highly refined researches on this subject have been made by Dumas, Stas, Roscoe, and Friedel, all tending to prove that the diamond is practically pure carbon. Chemists have generally experimented, for the sake of economy, with impure specimens, and have thus obtained on combustion a considerable amount of ash, the nature of which has not been well ascertained. It has been shown, however, that the purer the diamond the smaller is the proportion of ash left on its combustion. [Illustration: Square-cut Brilliant.] [Illustration: Round-cut Brilliant.] [Illustration: Rose-cut Diamond.] The art of cutting and polishing the diamond is said to have been discovered in 1456 by Louis de Berguem of Bruges. As now practised, the stone is first, if necessary, cleaved or split, and then 'bruted' or rubbed into shape. The faces of the stone thus 'cut' are ground and polished on flat metal discs, fed with diamond dust and oil, and revolving with great rapidity by steam-power. Antwerp comes first, then Amsterdam as the chief home of this industry, and the trade is chiefly in the hands of Jews; but diamond cutting and polishing are also now extensively carried on in London, Antwerp, &c. The common form of the diamond is either the _brilliant_ or the _rose cut_. The brilliant resembles two truncated cones, base to base, the edge of the junction being called the _girdle_, the large plane on the top is the _table_, and the small face at the base the _culet_; the sides are covered with symmetrical facets. The rose has a flat base, with sides formed of rows of triangular facets rising as a low pyramid or hemisphere; but this form of diamond is daily becoming less fashionable, and is therefore of comparatively little value. Although the term 'carat' is applied to diamonds as well as to gold, it does not mean the same thing. Used with regard to the metal, it expresses quality or fineness--24-carat being pure gold; and 22-carat equal to coined gold. But applied to the diamond, carat means actual weight, and 151-1/2 carats are equal to one ounce troy. India was formerly the only country which yielded diamonds in quantity, and thence were obtained all the great historical stones of antiquity. The chief diamond-producing districts are those in the Madras Presidency, on the Kistna and Godavari rivers, commonly though improperly termed the Golconda region; in the Central Provinces, including the mines of Sumbulpur; and in Bundelkhand, where the Panna mines are situated. At present the diamond production of India is insignificant. It is notable, however, that in 1881 a fine diamond, weighing 67-3/8 carats, was found near Wajra Karur, in the Bellary district, Madras. The stone was cut into a brilliant weighing 24-5/8 carats, and is known as the 'Gor-do-Norr.'