The system of classification, which includes not only ores, but all known Minerals, is not easy for the visitor to follow, and it is therefore convenient to indicate the positions in the gallery of those minerals--as, for instance, precious and ornamental stones, and metallic ores--which have an interest for all; for details, reference must be made to the Mineral Guide. Most of these Minerals occur as crystals, the forms of which can be referred to six systems of crystallisation. [Illustration: FIG. 58.--THE COLENSO DIAMOND, FROM SOUTH AFRICA. NATURAL SIZE.] [Sidenote: Native Elements.] I. In cases 1 and 2 are the native metals, as Copper, Silver, Gold, and Platinum; and non-metals, as Sulphur, Diamond, and Graphite. The large symmetrical South African “Colenso” Diamond (fig. 58), weighing 130 carats, presented by the late Professor John Ruskin, is worthy of special attention (case 1f). Models of some famous diamonds, including “The Cullinan Diamond,” the largest ever found (weight before being cut, 3025¾ carats, or about 1⅓ lb. av.) are exhibited. [Sidenote: Sulphides.] II. The next six cases contain Minerals which have mostly a metallic lustre and consist of metals in chemical combination with elements of the Sulphur or Arsenic groups. Argentite (3d) is an important Silver-ore, containing 87 per cent. of Silver and 13 of Sulphur. Blende (4b) is a valuable Zinc-ore, and contains 67 per cent. of Zinc and 33 per cent. of Sulphur. Galena (4e) is by far the most important ore of Lead (Lead 87, Sulphur 13, per cent.). Copper-glance (3e) is a common ore of Copper (Copper 80, Sulphur 20, per cent.). Cinnabar (3h) is the ore from which Mercury or Quicksilver is obtained (Mercury 86, Sulphur 14, per cent.). Pyrites (5d), one of the most common of Minerals, is a compound of Iron and Sulphur (Iron 47, Sulphur 53, per cent.). Erubescite (5e), Copper pyrites (5f), and Tetrahedrite, or Grey Copper-ore (7a), are valuable sources of Copper. [Sidenote: Chlorides, etc.] III. Common Salt, the chloride of the metal Sodium, is exhibited in case 8f, and Fluor-spar, a fluoride of Calcium belonging to the same division, begins at case 7e. [Sidenote: Oxides.] IV. The next division consists of compounds of Oxygen and includes most of the stony Minerals. Cuprite (10a), an important ore of Copper (Copper 89, Oxygen 11, per cent.), is at first ruby-red in colour, but becomes blackened by exposure to light. Spinel (10e), in its transparent varieties, is one of the precious stones; the deep red being the Spinel Ruby (less dense and less hard than the true Ruby), the rose-tinted the Balas Ruby, and the yellow or orange-red the Rubicelle of the jewellers: sometimes, it has a dark blue colour. On account of their hardness, the less valuable specimens are used for the jewelling of watches. Magnetite, or Magnetic Iron-ore (10f), the richest ore of Iron, of which it contains 72 per cent., is the natural magnet.