<P> Finding kimberlites requires persistence, and only a small fraction contain diamonds that are commercially viable . The only major discoveries since about 1980 have been in Canada . Since existing mines have lifetimes of as little as 25 years, there could be a shortage of new diamonds in the future . </P> <P> Diamonds are dated by analyzing inclusions using the decay of radioactive isotopes . Depending on the elemental abundances, one can look at the decay of rubidium to strontium, samarium to neodymium, uranium to lead, argon - 40 to argon - 39, or rhenium to osmium . Those found in kimberlites have ages ranging from 1 to 3.5 billion years, and there can be multiple ages in the same kimberlite, indicating multiple episodes of diamond formation . The kimberlites themselves are much younger . Most of them have ages between tens of millions and 300 million years old, although there are some older exceptions (Argyle, Premier and Wawa). Thus, the kimberlites formed independently of the diamonds and served only to transport them to the surface . Kimberlites are also much younger than the cratons they have erupted through . The reason for the lack of older kimberlites is unknown, but it suggests there was some change in mantle chemistry or tectonics . No kimberlite has erupted in human history . </P> <P> Most gem - quality diamonds come from depths of 150 to 250 kilometers in the lithosphere . Such depths occur below cratons in mantle keels, the thickest part of the lithosphere . These regions have high enough pressure and temperature to allow diamonds to form and they are not convecting, so diamonds can be stored for billions of years until a kimberlite eruption samples them . </P> <P> Host rocks in a mantle keel include harzburgite and lherzolite, two type of peridotite . The most dominant rock type in the upper mantle, peridotite is an igneous rock consisting mostly of the minerals olivine and pyroxene; it is low in silica and high in magnesium . However, diamonds in peridotite rarely survive the trip to the surface . Another common source that does keep diamonds intact is eclogite, a metamorphic rock that typically forms from basalt as an oceanic plate plunges into the mantle at a subduction zone . </P>

How long does it take for diamonds to form