Topaz occurs principally in pegmatite dykes and in cavities in granite, and is interesting to petrologists as a conspicuous instance of the result of the action of hot acid vapours upon rocks rich in aluminium silicates. Magnificent crystals have come from the extensive mining district which stretches along the eastern flank of the Ural Mountains, and from the important mining region surrounding Nertschinsk, in the Government of Transbaikal, Siberia. Fine green and blue stones have been found at Alabashka, near Ekaterinburg, in the Government of Perm, and at Miask in the Ilmen Mountains, in the Government of Orenburg. Topazes of the rare reddish hue have been picked out from the gold washings of the Sanarka River, Troitsk, also in the Government of Orenburg. Splendid pale-brown stones have issued from the Urulga River, near Nertschinsk, and good crystals have come from the Adun-Tschilon Mountains. Kamchatka has produced yellow, blue, and green stones. In the British Isles, beautiful sky-blue, waterworn crystals have been found at Cairngorm, Banffshire, in Scotland, and colourless stones in the Mourne Mountains, Ireland, and at St. Michael’s Mount, Cornwall. Most of the topazes used in jewellery of the present day come from either Brazil or Ceylon. Ouro Preto, Villa Rica, and Minas Novas, in the State of Minas Geraes, are the principal localities in Brazil. Numerous stones, often waterworn, brilliant and colourless or tinted lovely shades of blue and wine-yellow, occur there; reddish stones also have been found at Ouro Preto. Ceylon furnishes a profusion of yellow, light-green, and colourless, waterworn pebbles. The colourless stones found there are incorrectly termed by the natives ‘water-sapphire,’ and the light-green stones are sold with beryl as aquamarines; the stones locally known as ‘king topaz’ are really yellow corundum (cf. p. 181). Colourless crystals, sometimes with a faint tinge of colour, have been discovered in many parts of the world, such as Ramona, San Diego County, California, and Pike’s Peak, Colorado, in the United States, San Luis Potosi in Mexico, and Omi and Otami-yama in Japan. CHAPTER XXII SPINEL (_Balas-Ruby_, _Rubicelle_)