Spinel, like diamond, belongs to the cubic system of crystalline symmetry, and occurs in beautiful octahedra, or in flat triangular-shaped plates (Figs. 73, 74) the girdles of which are cleft at each corner, these plates being really twinned octahedra. The refraction is, of course, single, and there is therefore no double refraction or dichroism; this test furnishes the simplest way of discriminating between the balas and the true ruby. Owing to isomorphous replacement the value of the refractive index may lie anywhere between 1·716 and 1·736. The lower values, about 1·720, correspond to the most transparent red and blue stones; the deep violet stones have values above 1·730. Spinel possesses little colour-dispersion, or ‘fire.’ In the same way the values of the specific gravity, even of the transparent stones, vary between 3·5 and 3·7, but the opaque ceylonite has values as high as 4·1. Spinel is slightly softer than sapphire and ruby, and has the symbol 8 on Mohs’s scale, and it is scarcely inferior in lustre to these stones. Spinel is easily separated from garnet of similar colour by its lower refractivity. Spinels run from 10s. to £5 a carat, depending on their colour and quality, and exceptional stones command a higher rate. Spinel always occurs in close association with corundum. The balas and the true ruby are mixed together in the limestones of Burma and Siam. Curiously enough, the spinel despite its lower hardness is found in the river gravels in perfect crystals, whereas the rubies are generally waterworn. Fine violet and blue spinels occur in the prolific gem-gravels of Ceylon. A large waterworn octahedron and a rough mass, both of a fine red colour, are exhibited in the Mineral Gallery of the British Museum (Natural History), and a beautiful faceted blue stone is shown close by. The enormous red stone, oval in shape, which is set in front of the English crown, is not a ruby, as it was formerly believed to be, but a spinel. It was given to the gallant Black Prince by Pedro the Cruel after the battle of Najera in 1367, and was subsequently worn by Henry V upon his helmet at the battle of Agincourt. As usual with Indian-fashioned stones it is pierced through the middle, but the hole is now hidden by a small stone of similar colour.