The diamond is rarely found of a perfect shade of blue; but there are now in Europe several magnificent gems of this description. Foremost of all of them stood the famous blue diamond of 67²⁄₁₆ karats, belonging to the French crown. This marvel of Nature’s work, with two other diamonds of paler hue and lesser weight,--thirty-one and ten karats respectively,--disappeared on that fatal night of September, 1792, and have never since been discovered. At the present day, the finest known is that which belongs to the princely collection of the late Mr. Hope, and weighs 44¹⁄₄ karats. It is of a fine blue; but exhibits that steel-like tint which is so often seen in sapphires. The next in value and beauty is that which is preserved at Munich. It is a magnificent gem of thirty-six karats weight, and of superb color. The crystallized black diamond is a very rare stone; and, when polished, it forms a unique gem, since it exhibits a remarkable brilliancy, proceeding, as it were, from darkness itself. We do not now refer to the compact variety, known as carbon or carbonado, which is never found except in the amorphous form, but the crystalline variety, which is of greater density and more homogeneous. The famous collector, Dogni, possessed a very fine specimen of this kind which had been cut with small facets, and exhibited a vivid eclat. It afterwards came into the possession of Mr. Bapst, who disposed of it to Louis XVIII. for the sum of twenty-four thousand francs. A large and unique diamond, almost black, formerly belonged to the late Duke of York. Several of the European mineralogical cabinets have interesting and valuable collections of colored diamonds; but the finest is to be seen in the Imperial Cabinet of Minerals at Vienna. This beautiful and complete series, which illustrates the great range of the color-suite of the gem, was the life-long labor of a Tyrolese gentleman, by the name of Helmreicher. This enthusiastic amateur went to Brazil, and passed most of his life in the mines, searching for the gems.