Diamonds

Diamonds are the most popular and precious of all of the world's gemstones. Due to their exquisite beauty, which is largely a result of their ability to refract light, diamonds are often referred to as "the gem of gems". Diamonds also possess unique properties, most notably their exceptional hardness, that make them useful in industrial, medical and scientific applications.

All Rio Tinto Diamonds group mines operate at the highest levels of environmental, safety and social standards. Rio Tinto Diamonds is the only producer with more than one mine that distributes rough diamonds in separate product streams, providing both manufacturing and retail customers with confidence of mine and country of origin.

How diamonds are formed
Natural diamonds were formed approximately three billion years ago, 150 kilometres beneath the earth's surface. Under very high temperatures (between 900 and 1300 degrees Celsius) and great pressure, carbon atoms combined to crystallise and grow as diamonds within rocks.

During this time, molten lamproite and kimberlite (magma) were also forming within the Earth's upper mantle, and, subjected to similar pressures, were forced upwards through the diamond-bearing rocks. The magma partially melted the rocks and created a deep and narrow channel while carrying the diamonds towards the Earth's surface. As the magma came closer to the surface, it exploded and erupted at the surface to form a pipe. Over millions of years the lamproite and kimberlite ash within the pipes solidified and became diamond-bearing rock known as tuff. Both lamproite and kimberlite are magnesium-rich and contain minerals rich in chromium, titanium and potassium.

Diamonds of powder size can also be formed by shock waves produced from explosions. Such production has been achieved in the laboratory and also in nature where meteorites have struck the earth.

How diamonds are mined
Diamonds are mined using both open pit and underground mining.

The principal activity in open pit mines involves the removal of overburden (rocks and soil above diamond-bearing rock) and the collection of diamondferous ore. Ore in the pit is broken up by blasting, with each blast liberating about 3000 tonnes of rock.

Excavators capable of holding 45 tonnes then load the rock into haul trucks that have an average carrying capacity of 2000 tonnes. The trucks transport diamond-bearing ore to a primary crusher to commence the diamond extracting process.

Underground mining involves tunneling into the ground close to the diamond pipe to access the ore body. Tunnels are constructed on two levels, one above the other. Mining begins on the top level by blasting a slice of ore all the way across the ore body. Funnels are built to connect the top level tunnels with the bottom level so the broken rocks fall down to the tunnels below where they are collected by loaders. This method of underground mining is known as block caving.

How diamonds are processed
Ore from the mine is delivered to the process plant by haul trucks. Diamonds are liberated and recovered from the ore in six stages:

The mined material goes through a primary crusher that reduces it to a maximum size of 150mm. The secondary crusher's product is conveyed to the primary stockpile, which has a capacity of 10,000 tonnes.
The ore is extracted from the primary stockpile at a rate of 1,500 tonnes per hour. It is conveyed to high pressure rolls-crushers, where its maximum size is reduced to 30mm.
Ore is then scrubbed and screened, where it is separated into three sizes. Oversized material (larger than 15mm) is further reduced, and undersized material (smaller than 1.5mm) is rejected to the tailings dump, as diamonds in this undersized ore cannot be extracted profitably. Ore between 1.5mm and 15mm is conveyed to a Heavy Media Separation (HMS) Plant Feed Stockpile.
The majority of the material in the HMS stockpile is lamproite ore. It contains diamonds and some other high density minerals. The ore is then processed in a cyclonic separation plant. Heavy media consisting of ferrosilicon powder mixed with water is used to separate the lamproite ore from the diamonds and heavy minerals which sink to provide a diamond rich concentrate.
In the recovery plant, the diamond concentrate is fed through a series of custom-designed x-ray sorters. Diamonds fluoresce when exposed to x-rays. Sensors detect the flashes of light emitted by the diamonds. These send signals to the microprocessor that fires an air blaster valve at the appropriate moment, blowing the diamonds into a collection box.
The diamonds are finally acid-cleaned, washed, weighed and prepared for transport.

Diamond sales
The majority of diamonds are sold as "rough" or uncut stones. The diamond miles at Argyle, Diavik and Murowa provide customers with a wide range of rough diamonds.

Rio Tinto's diamonds are sold in large volumes on a willing buyer willing seller basis to a number of specialist customers. The relationship between Rio Tinto Diamonds and its customers is based on mutual understanding and trust, and is long-term in nature. Rough diamond sales are conducted ten times per year in line with the industry's market cycle. They take place in Antwerp where customers can view product and discuss their requirements. Antwerp is the global centre for the wholesale trade in rough diamonds.

Rio Tinto Diamonds also operate a niche polished sales business through Argyle Diamonds, where pink polished diamonds are produced and sold to an international market of diamond wholesalers, fine jewellerey manufacturers and high-end jewellery retailers.

Rio Tinto Diamond's Mumbai office focuses on diamond cutting and sales operations in India, where two thirds of the group's production by value is processed. Through its Mumbai office, Rio Tinto Diamonds has developed and implemented processes to ensure that diamond traders, jewellery manufacturers and exporters are provided with sales and technical support.

For more information on Rio Tinto Diamonds sales, visit:

The Select Diamantaire Mark
On June 3, 2006, Rio Tinto Diamonds introduced the Select Diamantaire Mark to the trade and press at the JCK Show in Las Vegas. The Select Diamantaire Mark is a registered trademark of Rio Tinto Diamonds N.V. and is licensed annually for use by Rio Tinto Diamonds' customers. The Mark recognises their status as providers of diamonds from Rio Tinto mines to the diamond and jewellery trade.

The Select Diamantaire Programme is a customer recognition programme that celebrates and promotes the distinctive skills of Rio Tinto Diamonds's core customers, each of whom are selected for their specific experience and expertise in the trading, cutting and polishing of diamonds.

For more information on our other products, see our corporate fact sheets.