The continental crust is old—up to four billion years old.

Its oldest parts, the ancient continental nuclei, or , are isolated in the interior of the continent by belts of successively younger continental crust (figure 4).

Archean cratons in South Africa have yielded gem diamonds such as these specimens from the GIA Museum’s Oppenheimer Student Collection. Since then, there have been significant advances in the analysis of diamonds and their mineral inclusions, in the understanding of diamond-forming fluids in the mantle, and in the relationship of diamonds to the deep geology of the continents and the convecting mantle.

This phase diagram depicts the stability fields of graphite and diamond in relation to the convecting mantle (asthenosphere) and the lithospheric mantle.

The graphite/diamond transition was recently revised to lower pressures (Day, 2012), providing for even greater storage of diamonds at shallower levels in the cratonic keel.

By comparison, oceanic crust is much younger and progresses regularly in age from zero (formation today) to the oldest known ocean floor, which is about 0.2 billion years old.

This basic age distribution of rocks at the earth’s surface (Hurley and Rand, 1969) became widely known within five years of the acceptance of plate tectonics theory in the mid-1960s, as naturally decaying radioactive elements (uranium, thorium, and rubidium) provided a quantitative way to measure the geologic age of exposed crustal rocks. World diamond localities are shown here in relation to Archean cratons and classified as either kimberlite-hosted and from mantle keels (lithospheric), kimberlite-hosted and from the convecting mantle (superdeep), of surface origin (alluvial), from ultra-high-pressure crustal terranes (UHP crustal), or formed by the shock of meteorite impact (impact).

The earth’s upper surface is composed of rigid, lithospheric plates of crustal rock (too stiff to flow on geologic time scales, yet stiff enough to break and cause earthquakes) underlain by mantle rock.

Surface deformation, volcanic activity, and earthquakes occur more readily at the margins of plates than at their interior.

While these pressure-temperature conditions seem extreme, for a large rocky planet such as ours, they are not.

Within the earth, temperature always rises with depth along a path known as the Figure 3.

The occurrence of natural diamonds is remarkable and important to earth studies.

This article reviews current thinking of where, how, when, and why natural diamonds form.

Localities are as follows: (1) Diavik, Ekati, Snap Lake, Jericho, Gahcho Kue, DO-27; (2) Fort a la Corne; (3) Buffalo Hills; (4) State Line; (5) Prairie Creek; (6) Wawa; (7) Victor; (8) Renard; (9) Guaniamo; (10) Juina/Sao Luis; (11) Arenapolis; (12) Coromandel, Abaete, Canasta; (13) Chapada Diamantina; (14) Boa Vista; (15) Koidu; (16) Kan Kan; (17) Akwatia; (18) Tortiya; (19) Aredor; (20) Bangui; (21) Mbuji-Mayi; (22) Camafuca, Cuango, Catoca; (23) Masvingo; (24) Mwadui; (25) Luderitz, Oranjemund, Namaqualand; (26) Orapa/Damtshaa, Letlhakane, Jwaneng, Finsch; (27) Murowa, Venetia, The Oaks, Marsfontein, Premier, Dokolwayo, Roberts Victor, Letseng-la-Terae, Jagersfontein, Koffiefontein, Monastery, Kimberley (Bultfontein, Kimberley, De Beers, Dutoitspan, Kamfersdam, Wesselton); (28) Kollur; (29) Majhgawan/Panna; (30) Momeik; (31) Theindaw; (32) Phuket; (33) West Kalimantan; (34) South Kalimantan; (35) Springfield Basin, Eurelia/Orroroo, Echunga; (36) Argyle, Ellendale, Bow River; (37) Merlin; (38) Copetown/Bingara; (39) Mengyin; (40) Fuxian; (41) Mir, 23rd Party Congress, Dachnaya, Internationalskaya, Nyurbinskaya; (42) Aykhal, Yubileynaya, Udachnaya, Zarnitsa, Sytykanskaya, Komsomolskaya; (43) Ural Mts.; (44) Arkhangelsk; (45) Kaavi-Kuopio; (46) W Alps; (47) Moldanubian; (48) Norway; (49) Rhodope; (50) Urals; (51) Kokchetav; (52) Qinling; (53) Dabie; (54) Sulu; (55) Kontum; (56) Java; (57) New England Fold Belt; (58) Canadian Cordillera; (59) Lappajärvi; (60); Ries; (61) Zapadnaya; (62) Popigai; (63) Sudbury; and (64) Chixculub. (2013), with permission of the Mineralogical Society of America. Plate tectonics is the modern unifying theory that explains the earth’s active geologic processes today, and is thought to have operated perhaps for as long as the latter half of the planet’s history.