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Diamond Gemstone Information

The name Diamond comes from the Greek word 'Adamas' the unconquerable, in recognition of the hardest naturally occurring material.

There are many famous diamonds, some with a long and unclear history. The largest cut diamond is the Cullinan 1, a pear-cut stone at 530.20 carats, named after Sir Thomas Cullinan, chairman of the Premier mine near Johannesburg. It was set into the British Royal Septre and remains on display in the Tower of London.

In the ancient past it would have been impossible to fashion diamonds due to their extreme hardness and a lack of knowledge on crystalline structures. Jewellery makers in Rome used octahedral shaped diamond crystals (shaped like double sided pyramids) to set into rings, they were called point stones. Slow development through the ages eventually arrived at the facet cut and polished diamond. Modern daydiamond cutting is still a long job, the process involves cleaving - slicing off sections, Bruting - basic girdle shaping by using another diamond as a lathe tool. Then cutting and polishing - using flat laps charged with differently graded diamond grit. Large diamond cutting firms with modern facilities, use lasers to cut diamonds. This method reduces the risk of some diamonds shattering due to internal stress and challenging inclusions. Also, reduced cutting times greatly increases productivity and therefore profit. More valuable stones are still cut by traditional methods as laser cutting incurs more wastage.

The most popular cut for diamond is the brilliant cut, of which several variations exist. The angles of the facets on the pavilion and crown, plus the size of the table facet are important factors when designing a stone. The size of the table facet determines the amount of brilliance and dispersion (fire) that the stone returns. A larger table facet increases brilliance and reduces dispersion and a smaller table has the opposite effect, with less brilliance and more dispersion. Fire refers to the spectral colours seen through the crown facets, where light is dispersed in the same way it is through a prism.

To achieve maximum brilliance the pavilion facets have to be cut to critical angles. This allows all of the light going into the diamond, through the crown facets, to be reflected back out again. This is known as T.I.R. - Total Internal Reflection.

There are now many fancy coloured diamonds on the market due to irradiation treatments, natural blue diamonds are very rare but readily available as enhanced stones.

Accent diamonds are small stones, commonly used to compliment other precious and semi-precious gemstones in jewellery rings and pendants.

Diamonds use the familiar four C's grading system: Colour, Clarity, Cut and Carat weight.

There are different systems in use, employing many parameters to derive at the correct diamond grade. Comparison stones are used to arrive at the correct colour grade for white diamonds, there are 10 in total (grades D to M). Modern comparison stone sets are made of cubic zirconia (CZ) and are very reasonably priced.

Major source locations for diamond include: Africa, Australia, Canada, China and Russia.

Physical Properties

Stone type:
Diamond.
Crystal System:
Cubic (isometric).
Chemical Composition:
Carbon, C.
Colour:
Colourless, off-white to yellow, pink, brown, black, rarely as green, blue, red or purple.
Lustre:
Adamantine: the highest lustre
Pleochroism:
None.
Dispersion:
High at 0.044
Hardness:
10 on the Mohs' scale of hardness, the highest of all gemstones and all known naturally occurring substances.
Toughness:
Good.
Cleavage:
Perfect: cleaves to produce totally flat planes.
Density in gm/cc:
3.52
Double refraction:
None.
Refractive index:
2.42

Common Treatments

Irradiation and heat treatments are used to clarify or colour stones. Various colours are now produced.

Laser Drilling is used to remove black inclusions, the resulting cavity is usually filled with glass. Natural cavities and surface depressions are also glass-filled.

High tech coatings have been applied to colour diamonds.

HPHT (high pressure high temperature) treatment is used to remove the brown colour from some diamonds by straightening a 'twisted lattice', the cause of the undesirable colour.

Diamonds may be manufactured using a HPHT method that employs seed diamonds and carbon material. They sell at a much lower price than natural diamonds.


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