Carbon is one of the most important elements on our planet, which led the Geological Society of London to name 2019 the Year of Carbon. Diamonds are a main host for carbon in the deep earth and also have a deeper origin than all other gemstones. Whereas ruby, sapphire, and emerald form in the earth’s crust, diamonds form many hundreds of kilometers deep in the earth’s mantle. Colored gemstones tell scientists about the crust; gem diamonds tell scientists about the mantle. This makes diamonds unique among gemstones: Not only do they have great beauty, but they can also help scientists understand carbon processes deep in the earth. Indeed, diamonds are some of the only direct samples we have of the earth’s mantle. But how do diamonds grow in the mantle? While Hol- lywood’s depiction of Superman squeezing coal captured the public’s imagination, in reality this does not work. Coal is a crustal compound and is not found at mantle pressures. Also, we now know that diamond does not prefer to form through direct conversion of solid carbon, even though the pressure and temperature conditions under which diamond forms have traditionally been studied experimentally as the reaction of graphite to diamond. Generally, two conditions are needed for diamond for- mation: Carbon must be present in a mantle fluid or melt in sufficient quantity, and the melt or fluid must become reduced enough so that oxygen does not combine with car- bon (see below). But do diamonds all grow by the same mechanism? What does their origin reveal about their growth medium and their mantle host rock? Surprisingly, diamonds do not all form in the same way, but rather they form in various environments and through varying mech- anisms. Through decades of study, we now understand that diamonds such as the rare blue Hope, the large colorless Cullinan, and the more common yellow “cape” diamonds all have very different origins within the deep earth. Diamonds Form from Fluids in the Mantle That Migrate Due to Plate Tectonics Diamond is a metasomatic mineral that forms during mi- gration of carbon-bearing fluids, which means that it forms from fluids and melts that move through the mantle. Dia- monds can form in both peridotite and eclogite (box A) in the cratonic lithospheric mantle (box B), as well as their higher-pressure equivalents in the much deeper transition zone and lower mantle (box B). Regardless of a diamond’s formation depth, many diamond fluids and melts appear to be related to the recycling of surficial material into the deep earth or to deep melting processes when tectonic plates split apart, or rift, to form new oceans. Both processes occur as part of the geologic cycles that accompany plate tectonics or, in ancient times, some type of pre-plate tectonics. Since diamonds come from deep and otherwise inac- cessible regions, they can be used to study many larger- scale tectonic processes in earth that cannot be studied any other way. Diamonds reveal processes such as early craton development, craton growth and stabilization processes, tectonic processes that can modify and destroy the cratonic lithosphere, and deep subduction (box B) into the lower mantle that may reintroduce volatile elements into the deep earth. This makes the study of diamond source fluids, as well as inclusions in diamond, a powerful way (if not the only way) to study the deep earth cycles of many ele- ments such as carbon, nitrogen, boron, sulfur, and oxygen. How Do Lithospheric Diamonds Form? Lithospheric diamonds (box B) often contain detectable ni- trogen, implying that they crystallize from carbon- and ni- trogen-bearing (C-N-bearing) fluids. Through the study of diamonds from many different localities, we now know that there are subtle differences in the compositions of these C- N-bearing fluids and melts. These differences manifest as changes in the type of carbon and nitrogen compounds con- tained in these fluids. “Oxidized” hydrous fluids and melts can contain CO 3 , CO 2 , and N 2 , whereas more “reduced” hy- drous fluids contain CH 4 , NH 3 , and minor H 2 . Traditional models for diamond formation from fluids in the mantle invoke either carbonate (CO 3 ) reduction or methane (CH 4 ) oxidation to remove the elements bonded to carbon. Both these mechanisms require some oxygen exchange with the surrounding rocks—peridotite or eclog- ite (box A)—at the site of diamond precipitation so that el- emental carbon can be produced to crystallize diamond. Sometimes peridotite has a limited capacity to exchange oxygen, and we now know that cooling of hydrous fluids containing methane (CH 4 ) and carbon dioxide (CO 2 ) is an alternative way to precipitate diamonds in these rocks (fig- ure 1; Luth and Stachel, 2014; Smit et al., 2016; Stachel et al., 2017). 440 DIAMONDS FROM THE DEEP GEMS & GEMOLOGY WINTER 2018 DIAMONDS FROM THE DEEP WINDOWS INTO SCIENTIFIC RESEARCH Karen V. Smit and Steven B. Shirey How Do Diamonds Form in the Deep Earth? © 2018 Gemological Institute of America GEMS & GEMOLOGY , VOL. 54, NO. 4, pp. 440–445. WN Dia from the deep.qxp_Layout 1 1/3/19 10:55 AM Page 440