Genesis of Diamonds

Lesson 1

It is still uncertain how diamonds originated; they were formed much deeper in the earth than any other gemstone. The magma (the mother liquid in which they were formed.) appears to have originated more than 200 km (120 miles) down, but diamonds were most probably not themselves formed at the source. Most diamonds are found in continental areas with a history of long periods of geographical stability. Diamonds were formed under conditions of great heat and pressure inside the earth surface where extreme temperatures and pressure exist, as long as 3.3 billion years ago. Diamond is the only gem formed from a single element – Carbon. Crystalline gem materials possess an orderly internal structure of atoms and bonds. Diamonds are composed of carbon atoms (like graphite in a lead pencil). Minerals are solid, inorganic, crystalline materials, which are formed by geological processes.

Most minerals including peridot, garnet, zircon and tourmaline have a characteristic composition and crystal structure. Gem quality diamonds are typically more than 99.95% pure carbon and the remainder is made up of over 25 different impurity elements. Diamond crystals with only 0.05% of the crystal weight as impurities are amongst the purest creations in nature. Diamonds crystallize in the crystal system of the highest symmetry – The Isometric System (The Cubic System)

Structure of Diamonds

The carbon atom is unique in having 4 electrons in an outer shell which can hold up to 8. It, therefore, occupies a position half-way between the electro-positive and electro-negative elements, yet is not chemically neutral. Two carbon atoms can link together by sharing an electron from one, which fills a hole in the shell of the other. As there are 4 electrons and 4 holes to each atom, each atom can link with 4 others; i.e. it has a valency of 4. Such bonds are called covalent bonding and can hold atoms together so that they form geometrical structures known as atomic crystals. The lattices are always strong and rigid and the resulting substances are hard solids with high melting-points. The most exceptional atomic crystal is that of a diamond.

Single molecules of such substances do not exist. Diamond can, therefore, be described as a giant saturated molecule of carbon. The 4 bonds to each atom are not, of course, flat, but are at 109.5 ° degrees to each other. Each atom is at the geometric centre of 4 others which are at the corners of an imaginary tetrahedron. Every bond is the same, 1.544 ångstrom units long. Definition of Ångstrom – Symbol Å (a unit of length equal to one hundred-millionth of a centimetre, used mainly to express wavelengths and interatomic distances). One layer is shown shaded to draw attention to the puckered hexagonal rings of atoms that result from the structure. This is, in fact, one octahedral direction in the crystal, the others being in the planes of a tetrahedron. In graphite, the only other crystalline form of carbon, the atoms form also into hexagonal rings, but the rings are flat and in thin plates. Within a plate, the atoms are closely linked with atomic bonds of 1.42 ångstrom units – more powerful even than diamond. Atomic forces between the layers are feeble, however, the bond being 3.35 ångstrom units, and they sheer easily, giving the substance a greasy feel.

VSEPR theory predicts that a carbon atom with four single, covalent bonds has a tetrahedral shape.


Almost every natural octahedral diamond face has triangular markings on it, known as trigons. Some trigons are easily recognizable by the naked eye and a face with visible trigons probably invariably has thousands of faint ones as well, when viewed under high magnification. Some may be as shallow as ten Ångstrom units. The triangles are equilateral – (meaning having all its sides of the same length) with straight edges. They are orientated oppositely to the face – their corners point to the edges of the face – and they are always aligned strictly with each other. Trigons appear to be of at least three types – pyramidal pits which go down to a sharp point, pyramidal pits with flat bottoms, and flat-bottomed pits with steep almost vertical sides. Trigons are commonly known in the diamond trade as growth marks.