Physical Properties of Diamond

Diamond possesses a number of very special physical pro­perties. From amongst these we select as relevant to our theme four important characteristics, under the headings (I) Mechanical, (2) Thermal, (3) Optical and (4) Electrical.

MECHANICAL PROPERTIES

Hardness

The first notable mechanical property which has already been touched upon is hardness. Now, every crystal (not only Diamond) has directional hardness characteristics. A crystal is a strict geometrical organization of atoms. These are locked -into a small three-dimensional group pattern, which, like the pattern on a wall-paper, repeats itself regularly an extremely large number of times. It is the unit atomic grouping which determines the final shape of the crystal. Thus ordinary table salt has a regular cubic atomic array and builds up accordingly into cubic blocks. The carbon atoms in graphite form a-hexagonal plate-like group, and this appears in the final shape of a graphite crystal. Sulphur crystals, sugar crystals and Diamond crystals all have a final pattern fixed by the small atomic unit group which multiplies up to make the crystal.

Now, without exception, the forces binding the atoms together can act with different strengths in different directions. Consequently, in every crystal in nature the physical properties vary in different directions. Directions in a crystal are recog- > disable from the placings of the crystal faces. It is usual to talk of crystallographic axes, meaning thereby directions in the crystal relative to the crystal planes. Because of this organ­isation of atoms in the pattern, the binding forces lead to ' specific different directional properties. Thus along the one crystal axis the mechanical strength, or the compressibility, will differ from that along a different crystal axis. The same is true of thermal conductivity, heat passing through usually more easily one way than another. Most important of all, from our point of view here, is the fact that the hardness differs

along (or across) different axes. This has been known for centuries and was perhaps discovered in India, maybe even 1000 years ago. Let us look at Fig. 3. Here we have a Diamond . octahedron, with eight similar triangular faces (one is shown shaded). These are called 'octahedral faces'. A number code system is used in crystallography to define faces, and such I triangular planes (all eight are similar) are described in this code as (111). In Fig. 4 we show the appearance of an octa- \, hedron which has been truncated to produce the square j

tabular shaded plane. Such a plane is called a 'cubic face', and ils code is (100).

Experience teaches us that the octahedral (111) face is the hardest' of all planes on a Diamond, whereas the cubic (100) lace is the 'softest'. This was discovered by accident and became the first secret of the Diamond polisher. In medieval limes this differential hardness became known to the guilds of lapidaries, who passed on the lore from father to son, holding it as a trade secret.

could abrade the softer, i.e. Diamond cut Diamond. This we shall go into later. The second consequence was that lapidaries were thereby able to grind down Diamond octahedra (and these were a common variety in ancient times) to the shape shown in Fig. 4. This became known as the 'table cut'. It is attractive ;ind splendent and is to be found in numerous pieces of medieval jewellery - in rings and brooches, for instance. Actually it is particularly elegant in use for the decoration of crosses, wherein if the table-cut GEMS are set along the arms of the cross, this produces a most pleasing aesthetic appearance. It was later that lapidaries began to attack the edges of the table cut, and this in due course led to shapes called the ‘brilliant cut’, one of many versions of which is shown in Fig. 5. It is easy to see how this historical evolution from octa­hedron to brilliant took place. There were, in fact, two guiding principles behind this development. One was to retain as, much of the original octahedron as possible in the final gem, the other was to secure maximum light brilliance, of which we shall speak later.

Cleavage

Diamond, like a number of other crystals, has one very important physical strength characteristic which plays an absolutely; fundamental part in the technological use of Diamond in industrial practice. This is the fact that despite its great hard­ness, Diamond cleaves with little difficulty. Crystalline cleavage is a special property shown to varying degree by different crystals. Some hardly cleave at all. Others, such as mica, cleave so easily that perfect crystal sheets can be peeled off of extensive area and of exceptional thinness, yet quite strong. In many crystals the organisation of atoms is such that in certain directions the binding forces are uniformly a good deal less than in other directions. These looser-bound planes arc the cleavage planes.