Today we know, specially through the experience of producing of synthetic diamonds, that diamonds are formed by nature in the interior of the earth under very high pressure of about 50.000 a.e.p. and temperatures above 1000 degrees centigrade. Such conditions exists in the interior of the earth in a depth of some hundred kilometres in the zone of so-called „Upper coat“.

Primary deposit

The diamond came to the surface of the earth through volcanic activity about 70 - 150 million years ago. This volcanic rock named „Kimberlit“ is poor of silicium but rich of mangan and ferrum. The from the volcanic eruption created accumulation (in the cone of the volcano) was taken down through the erosion , so that in the end a kind of cut-off chimney of „Kimberlit“ was left over - this is called a „ pipe “. Because of its sometimes bluish colour the "Kimberlit“ also named " blue ground ". Is this " blue ground " weather-beaten in the upper layer of the pipe, it's called " yellow ground ". At the highest layers of the " pipe " the "Kimberlit" is mined by opencast or in major depth by tunnels.

Secondary deposit

The erosion of volcanic pipes through water, wind, strong variation of temperature and other effects result in a transport of the diamondcontaining rock through streams and rivers to new deposits. This new, second deposit is called " secondary ". This secondary deposits exists also as coarse-grained sediments (in dried up streams or rivers) or on the seabed like in Namibia. During the transport the accompanien rock will be crushed, while the diamond resits various claims because of its hardness. Specially "closed" crytals without damagings resist the transport to the secondary deposits, called "soaps". Often there we found a lot of rough diamonds especially big and in good shape. The part of the production which is qualified for diamonds for jewels is higher at the secondary deposits. In South-Africa there are about 80% of the diamonds from primary, 20% from secondary deposits - in Namibia there are only secondary deposits. For to get one carat rough diamonds from a primary deposit (pipe), there are aproximately 4-5 tons of rocks to extract and treaten. The productivity is from mine to mine very different, refering the quantity and quality of the diamonds. The following table is a view about the productivity per ton for some mines in South-Africa.

Literary reference
Two books that are a must in the branche
Diamantenkunde mit kritischer Darstellung der Diamantengraduierung - Godehard Lenzen
Diamanten-Fibel - Verena Pagel-Theisen