• Diamonds are minerals that are completely made of the element carbon.


• Diamonds are the hardest known substance on Earth


• The heat and pressure required to form a diamond from carbon is present deep in the Earth.


• Diamonds form from magnesium-rich melted rock. Scientists believe at a depth of more than 93 miles (150 km) below the surface, the magnesium-rich rock is saturated with carbon dioxide gas at a temperature over 2600 F (1400 C) to form the crystal structure of the diamonds. At this depth there is a great amount of pressure.


• Geologists believe that eruptions of carbon dioxide gas deep below the Earth's crust push deep mantle and shallow mantle material up into the Earth's crust, mixing together until it cools. This material is known as kimberlite. Kimberlite also contains diamonds. Kimberlite is found in chimney-shaped deposits that sometimes reach the surface.


• Diamonds have extremely high thermal conductivity. Its thermal conductivity is five times greater than copper.


• Diamonds are measured by a unit called carats. This is a metric unit which is equal to 0.20 grams or 0.00704 ounces.


• The largest diamond discovered so far is the Cullinan diamond, named after the person who opened the mine where it was found, Sir Thomas Cullinan. It was 3106 carats (over 620 grams) when discovered in South Africa. It was cut into 9 large diamonds and various other smaller diamonds. The largest piece is the Star of Africa, a 520 carat diamond which was given to King Edward VII of Great Britain was placed in his royal scepter. The Star of Africa is the 2nd largest cut diamond to date and is on display at the Tower of London.


• When diamonds are cut for use as a gemstone, they are cut in order to maximize value, not size.



The most popular use of diamonds is for gemstones in diamond rings and other jewelry.



• Besides using diamonds for gemstones, diamonds also have industrial use. Diamonds are very useful for cutting because they are so hard. Drill bits and cutting wheels with bonded pieces of diamonds easily cut through steel, brick, concrete and other hard materials. Diamonds were also previously used on phonograph (record player) needles.


• Synthetic diamonds are also available. These diamonds are expensive to make, but are almost identical to natural diamonds.

Resources:

Hope Diamond Story - Smithsonian