In the pure state corundum is colorless, like pure diamonds which are also colorless.
Continuous weathering and erosion of the rocks over millions of years released the corundum crystals which were washed down the hills and deposited in the flood pains of rivers as alluvial deposits. Later as the igneous rocks were transformed into sedimentary and metamorphic rocks the corundum was also incorporated into these rocks. Subsequently, when the magma cooled down it formed igneous rock deposits and the corundum was incorporated in these deposits. Like diamonds, corundum was formed billions of years ago deep inside the earth, below the earth’s crust and in the mantle, under conditions of high temperature and pressure. Corundum crystallizes in the hexagonal system, resulting in pyramidal or rounded barrel shapes. The black star sapphire like all other sapphires belong to the group of minerals known as corundum, a crystalline form of aluminum oxide (Al2O3), that is after diamond, the hardest known natural substance, with a hardness of 9 on the mohs scale. Like all other star sapphires the star moves with changing angles of illumination and observation. The color contrast is indeed breathtaking. The beauty of the stone is further enhanced by its white gold setting in which the large black stone is surrounded by a row of 35 pear-shaped smaller white diamonds. The appearance of the bright six-rayed star in a totally black background is amazingly beautiful.
However, the center of the star is much bigger and brighter than the normal star sapphires. The rays are not of equal length some are shorter than the others. On the other hand, the rays of this black star sapphire are shorter and thicker extending about half way from the center towards the girdle. The rays in blue star sapphires are also fairly thin and narrow and tapers towards the girdle. The 733-carat Black Star sapphire is an oval-shaped gemstone cut as a cabochon like all other star sapphires, but the rays or legs of the star are not as long as the normal blue star sapphire and other colored star sapphires, which extend from the center to the girdle.
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