I wonder if this is even possible to know....
Do geophysicists and geologists have even a 'theoretical' idea about how deep Earth's churning of its layers goes?
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Diamond is stable at high temperatures only under very high pressures. Mineral inclusions in a diamond, and the rocks in which a diamond are embedded, can offer further clues as to the depth from which the diamond came.
The Wikipedia article on diamond claims "A smaller fraction of diamonds (about 150 have been studied) come from depths of 330–660 km, a region that includes the transition zone. They formed in eclogite but are distinguished from diamonds of shallower origin by inclusions of majorite (a form of garnet with excess silicon). A similar proportion of diamonds comes from the lower mantle at depths between 660 and 800 km. " (my emphasis)
As far as I know the best estimate is "upper mantle" from mantle xenoliths but that's not terribly specific. I am not sure if knowing more than that is possible right now.