Diamonds, when absolutely limpid and free from flaws, are said to be of the ‘first water,’ and are most prized when devoid of any tinge of colour except perhaps bluish (Plate I, Fig. 1). Stones with a slight tinge of yellow are termed ‘off-coloured,’ and are far less valuable. Those of a canary-yellow colour (Plate I, Fig. 3), however, belong to a different category, and have a decided attractiveness. Greenish stones also are common, though it is rare to come across one with a really good shade of that colour. Brown stones, especially in South Africa, are not uncommon. Pink stones are less common, and ruby-red and blue stones are rare. Those of the last-named colour have usually what is known as a ‘steely’ shade, _i.e._ they are tinged with green; stones of a sapphire blue are very seldom met with, and such command high prices. [Illustration: FIGS. 57—59.—Diamond Crystals.] Diamond crystallizes (Figs. 57—59 and Plate I, Fig. 2) in octahedra with brilliant, smooth faces, and occasionally in cubes with rough pitted faces; sometimes three or six faces take the place of each octahedron face, and the stone is almost spherical in shape. The surfaces of the crystals are often marked with equilateral triangles, which are supposed to represent the effects of incipient combustion. Twinned crystals, in which the two individuals may be connected by a single plane or may be interpenetrating, a star shape often resulting in the latter case, are common; sometimes, if of the octahedron type, they are beautifully symmetrical. The rounded crystals are frequently covered with a peculiar gum-like skin which is somewhat less hard than the crystal itself. A large South African stone, weighing 27 grams (130 carats) and octahedral in shape, which was the gift of John Ruskin, and named by him the ‘Colenso’ after the first bishop of Natal, is exhibited in the British Museum (Natural History); its appearance is, however, marred by its distinctly ‘off-coloured’ tint.