The variety of diamond called boart, or bort, deriving its name from the supposed abortive attempt of nature to form a perfect crystal, is also quite deficient in cleavage, or its laminæ are so irregular as to render splitting quite impossible and the cutting of the stone equally so. The transparency of these forms is also affected by the arrangement of crystallization; hence they are generally crushed into powder for polishing material or used for various purposes in the arts. The specific gravity of these varieties seems to be influenced by the manner of crystallization. For instances, we find that the fine transparent crystals have a specific gravity of 3.55 (water being considered the standard as 1), while the bort is somewhat less; and the massive variety called the carbonado varies from 3 to 3.4, according to the amount of earthy matter it may contain. It has also been asserted that the blue, the green, the orange, and the red varieties are heavier than the white. The phenomena of electricity observed in the diamond are not remarkable, and are inferior to most of the gems. Some of the precious stones when excited retain their electrical properties for hours or even days, but the diamond loses it almost immediately. It exhibits vitreous electricity when rubbed. Much has been said and written concerning the artificial phosphorescence exhibited by the diamond when removed to a dark room after having been exposed for a short time to the sun’s rays. We are not able to verify this statement, and feel inclined to doubt its correctness, although we have been assured by experimenters of the fact. One of the most remarkable properties of the diamond is its extreme hardness, in which it far exceeds all known substances in the mineral kingdom. This peculiarity is due to the substance itself, but appears to be modified by its color and its form of crystallization like some other minerals. The more perfectly the crystal is formed, the easier its laminæ become detached, and the softer the substance appears to be. In the globular forms, which are quite deficient in cleavage planes, the hardness is excessive, and often resists the most determined efforts of the lapidary. Even in fine crystals we shall find that certain angles are harder than others; and we may observe the same relative degree among crystals of other minerals, like those of the topaz. In the large transparent diamonds of irregular form, spots of excessive hardness are often found. These are called by the lapidaries “knots,” and appear to be due to a change in the process of crystallization. The coloring matter, or the mode of its formation, seems to affect the degree of hardness in many minerals; and in respect to the diamond, the rare crystallized black form is harder than the limpid or lighter colored.