Diamond

Diamonds formed around 1 to 3 billion years ago at depths of about 150 to 180 kilometres in the Earth’s mantle. They consist of pure carbon, which crystallises in cubic form under extreme pressure and high temperature.

Diamonds frequently crystallise as octahedra. Their faces can show typical triangular etch marks, the so-called trigons.

The long journey some diamonds make is also astonishing: the Orange River in South Africa and rivers along the coast of Namibia carry diamonds over enormous distances to the sea. There they accumulate in secondary deposits along river courses and coastlines as pebbles or in sediment.

At 3,106.75 ct, the Cullinan heads the list of the largest diamonds ever discovered. In 2024, an exceptionally large rough diamond of 2,492 ct was found in Botswana.

With a value of 10, diamonds have the highest hardness on the Mohs scale. Alongside colourless diamonds there are rare fancy colour diamonds. Famous examples are the blue Hope Diamond and the intense pink diamonds from the Argyle mine.

History & famous diamonds

In the 17th century, the French king Louis XIV owned the famous “French Blue”. The stone disappeared during the French Revolution. Decades later, a blue diamond surfaced in London that later became known as the Hope Diamond.

The Hope Diamond entered the collection of the Hope family in the 19th century. In 1958, the American jeweller Harry Winston donated the famous blue diamond to the Smithsonian Institution in Washington D.C., where it remains on display to this day.

Grading a diamond, the 4Cs

The internationally recognised 4C method describes the most important quality criteria of a diamond:

• Carat (weight): 1 ct = 0.20 g
• Colour: for colourless diamonds from D to Z
• Clarity: from FL/IF to I3
• Cut: decisively influences brilliance, fire and proportions

Fancy colour diamonds show intense and rare hues, from pink and yellow to blue or green. Clarity is assessed under 10x magnification. The international grading scale is: FL, IF, VVS1, VVS2, VS1, VS2, SI1, SI2, I1, I2, I3.