THE diamond, although not the most valuable of precious stones, yet unquestionably exceeds all others in interest, importance and general noteworthiness. In hardness, in the perfection of its clearness and transparency, in its unique constants of optical refraction and dispersion, and finally in the marvelous perfection of its luster, the diamond surpasses all other minerals. For these reasons, and despite the fact that it is not of very great rarity even in faultless specimens of fair size nine-tenths of the yearly trade in precious stones being concerned with diamonds alone it is very greatly valued as a gem, moreover, on account of its extreme hardness, it has several technical applications.
CHARACTERS OF DIAMOND.
Diamond is distinguished from all other precious stones no less by its chemical composition than by its unique physical characters, for no other gem consists of a single element. It is pure crystallized carbon. Its substance is therefore identical chemically with the material of graphite and charcoal. The extraordinary difference in the appearance of diamond and that of other forms of carbon depends solely on the crystallization of the material and the physical characters consequent on this.
The fact that the one and only constituent of diamond is pure carbon was already known at the end of the eighteenth century, and was suspected even earlier than this. In the year 1675 Sir Isaac Newton had arrived at the conclusion that diamond must be combustible, this conclusion, though correct in itself, was based on theoretical grounds, now known to be mistaken, connected with the high refractive index of the substance. In 1694-5, the Academicians, Averani and Targioni, at the instigation of the Grand Duke Cosmos III of Tuscany, conducted research, on the combustibility of diamond at the "Accademia del Cimento" of Florence. Diamonds were exposed to the intense heat of a fierce charcoal fire or were placed in the focus of a large burning-glass. A stone so treated did not fuse but gradually decreased in size and finally disappeared, leaving behind no appreciable amount of residue. These experiments proved that the substance of diamond, as such, is destroyed at a high temperature, whether its disappearance was due simply to volatilization, as in the case of ammoniac, was of course undecided at that early date.
The diamond is one of the most perfectly crystallized of minerals. Almost every single stone is bounded regularly developed faces. Massive specimens without crystal-faces are scarcely ever found, and when such are met with they are, as a rule, fragments of large crystals or rounded pebbles, of which the original external crystalline form has been destroyed. As is usually the case with embedded crystals, that is, those which have grown embedded in the mother-rock, most diamonds are bounded on all sides by crystal-faces. Sometimes, however, irregular areas, by which the crystal might have been attached, can be made out with more or less certainty.
The faces of diamond crystals differ from those of most other crystallized minerals, in that they are, as a rule, much curved and rounded instead of being perfectly plane, as is usually the case. This curvature is due to the mode of growth of the crystal, and not to subsequent attrition, as might be thought. It renders the exact determination of a crystal, what follows, the most important general crystallographic relations will be dealt with, while special features peculiar to diamonds from particular localities will be mentioned under the description of these localities.
Observations on the crystalline form of diamond date back to the beginning of the seventeenth century, many diamond crystals having been described by Keppler, Steno, Boyle, and others. Rome de l'Isle and Hauy, the founders of scientific crystallography, were, at the beginning of the nineteenth century, the first to correctly interpret the different forms, and to determine the hemihedral development of the crystals. Great credit is also due to Gustav Rose for his exhaustive study of diamond crystals made at a later date.
Crystals of diamond belong to the cubic system, and, according to the views of the majority of mineralogists, to the tetrahedral-hemihedral division of this system. Certain peculiarities, however, render the hemihedrism of the crystals open to question, and some authorities prefer to consider them as holohedral. All the typical simple forms of the cubic system have been observed in diamond crystals, either alone or in combination with other forms. Some of the more commonly occurring forms, which will be described in some detail, are shown here.
Crystals having the form of a regular cube (Fig. A) occur very frequently, but are usually small. This habit is specially characteristic of Brazilian and Congolese crystals, and is rarely met with in specimens from other localities, especially the Cape. The faces of the cube are always dull and rough, and show a shallow depression, increasing in depth towards the center of the face. The roughness is due to the presence of square-based, pyramidal depressions placed diagonally on the cube face, these are usually small, but may be of fair size. They occur more or less isolated or closely aggregated together (Fig. A). When observed with a lens or, better still, under the microscope, the pyramidal faces bounding the shallow depressions may be distinctly seen, they are plane and smooth, but just as frequently rough and irregular, and between these two extremes all gradations have been found.
In most cubes of diamond, however, each edge is replaced by two faces, as shown in (Fig. A), the twenty-four faces thus derived, would, if produced or enlarged sufficiently, give rise to the form known as the four-faced cube, or tetrakis-hexahedron. These faces are, however, as a rule, small, they are dull and uneven, and are irregularly striated perpendicularly to the cubic edges. Each face is often divided centrally by a narrow furrow, running perpendicularly to the edge and towards the center of the cube face, this is illustrated in (Fig. B), which shows one cubic face together with the four adjacent faces of the four-faced cube. A crystal, bounded only by the twenty-four faces of the four-faced cube and with no cube faces, is occasionally met with in diamonds from Brazil and India, the faces of this form are then bright, but always curved.
The cube is also frequently modified in ways other than by the replacement of its edges. Not infrequently, for example, its eight corners are truncated by the eight faces of the octahedron. Moreover, each of the twelve edges of the cube may be replaced by a single plane face, these twelve truncating faces, if extended, would give the form known as the rhombic dodecahedron, which is of frequent occurrence, and is shown in (Fig. C) and (Fig. I). Its faces are sometimes plane and striated parallel to the longer diagonal (Fig. I), as a rule, however, they are more or less curved, the lines of intersection of the faces being of course also curved, in this latter case the faces are not striated but are smooth and bright (Fig. C). These curved faces frequently have a shallow groove running across them in the direction of the shorter diagonal, as indicated by the dotted line in Fig. I, the form is then, strictly speaking, no longer a rhombic dodecahedron, but approaches to that of a tetrakis-hexahedron. The largest Brazilian diamond yet found, and known as the "Star of the South", is an irregularly developed rhombic dodecahedron. This form is frequently to be met with in Brazilian diamonds.
When the faces of the rhombic dodecahedron are grooved in the direction of the longer diagonal as well as in that of the shorter (Fig. D), we obtain a form known as a hexakis-octahedron, bounded by forty-eight similar faces, which are always strongly curved, smooth, and bright. The hexakis-octahedron, which is of extremely frequent occurrence in diamond, approaches, as shown in (Fig. D) , the form of the rhombic dodecahedron, at other times the same kind of form may approximate to the octahedron, each of the eight octahedral faces being replaced by six faces. Hexakis-octahedral crystals of the diamond are frequently much distorted by elongation in one direction, as shown in (Fig. F), a still greater distortion of the same form being represented in Fig. I. Such distorted forms, which appear at a first glance to be quite distinct from that of Fig. D, on a closer examination will be seen to be easily derived from that form.
Both the rhombic dodecahedron and the hexakis-octahedron are sometimes, on account of the strong curvature of their faces, almost spherical in shape. Formerly, when the principal localities for diamond were Brazil and India, the spherical form was known as the Brazilian type, and the octahedral form as the Indian type.
Rafal Swiecki, geological engineer email contact
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