Rare 93-carat natural black diamond goes on display in The Vault

The black stone has been carefully cut, polished, and set into an impressive white gold pendant. The Arabic-inspired design includes 41 colourless and 26 yellow diamonds on the rear, depicting the Al-Dubb al-Akbar constellation, also known as Ursa Major.

The diamond has been lovingly named 'The Anastasia Diamond' by the lender after his daughter.

Natural black diamonds are extremely rare. Only one in 10,000 natural diamonds is classed as fancy coloured, and out of these, only a handful are truly black.

Adding to the rarity is the very large size of this diamond, weighing an impressive 93 carats, which makes it larger than the famous Black Orlov Diamond. It is the largest diamond currently on display in the Museum.

Robin Hansen, gems curator at the Museum says, 'This gemstone appears black because it is made up of tiny individual diamond grains that are intergrown, known as polycrystalline. It also contains many minute inclusions of other minerals, mostly black graphite.

'The boundaries between the different grains, as well as these inclusions prevent light from passing through the diamond, so although it is partly translucent, it appears opaque black. The inclusions also create a glitter-like effect inside the diamond.'

The rough diamond, which weighed over 300 carats (60 grams), is thought to have originated in Brazil. It was acquired by the lender's family during the late 1800s in Goa, India, which was well known as a gem cutting centre at the time. The gemstone has remained with the family since.

How are diamonds formed?

Diamonds are a mineral composed of the element carbon. Forming under intense heat and pressure deep in Earth's upper mantle, diamonds were bought to the surface by fast and violent volcanic eruptions which travelled between 30-50 kilometres per hour.

These eruptions occurred millions of years ago and were unlike the volcanoes and volcanic eruptions we see today. They created underground tunnels to the surface known as kimberlite pipes, named after the South African town of Kimberley, where the first kimberlite pipe was discovered.

The speed at which these eruptions brought the diamonds to the surface was critical for their formation. The diamonds were carried to the surface often within a matter of hours, otherwise, they would instead have formed the mineral graphite.   

During their growth deep underground, diamonds may incorporate other minerals inside them, known as inclusions. The presence of these inclusions is what determines how clear a diamond looks.

Diamonds with no inclusions allow light to pass through easily, giving them a crystal-clear look with lots of sparkle. These diamonds are known as flawless and are some of the most sought after.

Diamonds containing inclusions are often not as appreciated by the jewellery trade, however, for scientists, these inclusions are incredibly valuable.

'Scientifically, diamonds are really important,' says Robin. 'The youngest diamonds known are around 660 million years old, and incredibly the oldest ones are several billion years old. They are much older than the kimberlite rocks in which they are found.

'Diamonds provide a snapshot of the inner Earth which we can never visit. They act like time capsules containing information from millions of years ago and help tell us about the history of our planet.'

What causes the diamond’s black colour?

Black diamonds have become popular in recent years. Whilst they don't have the same sparkle and fire of traditional diamonds, they are polished to promote the incredibly bright lustre of diamond, which is known as adamantine.

Black diamonds are also challenging to cut and polish because of the many inclusions, which makes this black diamond even more spectacular due to its incredible size.

Natural black diamonds - also referred to as fancy black diamonds - can be black for two main reasons.

Firstly, they may be polycrystalline, which means they are made up of many tiny intergrown crystals. They may also contain a high number of inclusions of minerals such as graphite and iron oxides or have internal fractures. These features all prevent light from passing through the gemstone, giving a dark, opaque appearance.

Secondly, diamonds can be irradiated to produce a dark colour. This can occur naturally underground if they are near rocks containing radioactive minerals, but it can also be artificially produced as a treatment by humans. This creates a green colour so dark that it appears black when the diamond is faceted into a gemstone.

The mystery of carbonado diamond

Carbonados are another type of natural black diamond. They are unique as they do not occur in kimberlite pipes and are not thought to form in Earth's upper mantle. Instead, they are found in sedimentary rocks from in Central African Republic (CAR) and Brazil.

These diamonds are polycrystalline, in a porous aggregate of tiny crystals. This polycrystalline nature gives them a higher toughness than other diamonds with excellent abrasive properties, meaning that they were often used for industrial purposes.

Carbonados range from opaque light grey to pure black, with a metallic looking lustre. The dark colour is similarly caused by the grain boundaries and inclusions.

Carbonados are over two and a half billion years old. This was in the pre-Cambrian period when CAR and Brazil were joined together as part of one supercontinent.

The formation process of carbonados is still a mystery. Some scientists believe the supercontinent experienced a meteorite impact creating the high pressure and temperature conditions required to form the carbonado diamonds.

Other scientists think the carbonados may actually have extra-terrestrial origins and arrived on Earth from space. This is because they lack inclusions of minerals found deep in the Earth like other diamonds, instead containing nitrogen, hydrogen and the mineral osbornite which is found in meteorites.

The Museum houses a vast number of mesmerising minerals with interesting backstories, many of which are on display. Visitors can see these stones and learn all about them.