In the remaining six window-cases on the same side of the gallery, a corresponding series of specimens illustrative of the characters and classification of Rocks is exhibited; and the eleven window-cases on the opposite side contain a collection illustrating the various kinds of Rocks.[22]
General Collection of Minerals.
In the table-cases of the gallery are exhibited specimens of every important Mineral species and variety preserved in the Museum. The cases containing this general collection are numbered from 1 to 41, and the eight panes of each case are severally distinguished by the letters a to h. For the use of the student there is published an Index to the names of all the numerous Mineral species and varieties represented in the collection, with references to the table-cases in which the specimens are placed.[23]
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.
Fig. 58.—The Colenso Diamond, from South Africa. Natural size.
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.
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.