Recently the distinguished savant Von Tschudi, in visiting some of the diamond mines of Brazil, was unable to distinguish readily the gems as they lay in the washed cascalho, while the trained eyes of the negro miners picked them out with ease. It has been stated that the diamonds are always or nearly always covered with an earthy crust of various hues, especially greenish or yellowish, which is hardly the fact; for what appears to be a crust is often caused by the salient edges of the laminæ, among which a little earthy or coloring matter has been introduced. These extraneous colors generally disappear when the surface of the stone is removed; and, in fact, the degree of their intensity is very much modified when the rough gem is placed in alcohol or in any fluid of high refraction.
The dull, whitish appearance seen in the natural diamonds is also produced by the action of fire, which raises the edges of the laminæ, producing a faint milky aspect.
The primitive form of the mineral is the octahedron, and many irregular masses may be reduced by cleavage to that of a double pyramid. These octahedral crystals are sometimes as perfect in outline as a mathematical model, with clean-cut angles and smooth faces. In some specimens the edges may be truncated, that is, as if they had been flattened or ground off by mechanical means. Generally, however, the crystals are of the form of the octahedrons with rounded faces. The dodecahedrons, with their twelve faces, and the cubes, with their four sides, may also be reduced by cleavage to the primitive form of the double pyramid. Sometimes two crystals are united, forming what are called hemitrope or twin crystals. Then, again, a number may be grouped together, assuming on the whole a globular-like mass. But they are decidedly different from the globular, which in their form of crystallization radiate from the centre of the crystal.
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.
Some years ago a black diamond from Borneo was placed in the hand of Gallais the lapidary, to be experimented upon at the expense of the French Institute. The chief object of the test was to ascertain the relative degree of hardness in comparison with some of the other varieties of diamond. In this trial the lapidary wore out his steel wheel and a large quantity of ordinary diamond dust without making the least impression on the surface of the black diamond. Although heavily loaded with weights, it lost none of its roughness, and was heated almost to whiteness by the friction of the wheel, which revolved with great velocity. During the period of this extreme velocity it is reported that a shower of sparks was emitted; but how shall we account for this scintillation, when the ordinary transparent diamond does not give forth sparks when struck by steel?
The carbonado, which is amorphous and without cleavage, is also extremely hard. The term “adamas,” which the ancients bestowed upon it as denoting an invincible infrangibility, is not quite appropriate; for although it is far superior in hardness to all other known substances, it is in reality very fragile. And in the power to resist the effect of shock it is also inferior to some of the other gems, and especially the sapphire. Therefore several mineralogists have thought that the ancients really applied the term to steel or to some of the varieties of corundum, like the ruby and the sapphire, and not the diamond. It is curious that this property should be ascribed erroneously to the diamond for so many ages, when a trifling experiment would have disclosed the real condition of things. In the days of the poet Lucretius the gem was believed to be able to resist violent blows.
——“adamantina saxa
Prima acie constant, ictus contemnere sueta.”