Surprising as it may seem, ruby and sapphire, so different in colour, actually belong to the same mineral family: corundum, the mineral form of alumina which crystallizes in the hexagonal system. The red colour of ruby results from a small admixture of chromic oxide. The most prized tint is blood red or crimson known in the trade as "pigeon's blood" red. The colour of this corundum varies, however, according to the geographical locality of the mine. Siamese rubies, for instance, are of a deeper garnet red than those found at Mogok in Burma, while Ceylon ruby is of a pinkish dull red. Corundum is extremely hard, 9 on the Mohs scale. It is, therefore, not quite as hard as diamond but much harder than other minerals, including emerald. Transparent ruby, when cut en cabochon, may reflect light so as to produce star-like bands, a phenomenon rated highly by the experts. If viewed from a certain direction, bands of light are reflected onto the surface of the crystal forming a six-ray star shape. This optical effect is due to the presence of fine canaliculi or inclusions of rutile. The colour of these "starstones" varies from pale rose red, to deep crimson, to purple. Generally speaking, the darker the crystal, the less evident is the star, and vice versa. It is a rare gem indeed in which the colour and the star are of equal beauty.