This is a sulphide mineral inclusion trapped in a diamond from Ellendale (Australia). The sulphides were broken out of the diamonds, dissolved, and after some chemical separation procedures in the lab, rhenium and osmium isotopes were measured, from which it was determined that the Ellendale diamonds are 1.4 billion years old. Image by Karen Smit.
To determine the absolute ages of rocks and minerals such as diamond, scientists measure naturally occurring radioactively decaying elements. Absolute ages are free of any knowledge of relative age relations to any other geological material. This is known as the science of geochronology.
Though these dating studies, we have shown that the oldest diamonds on Earth are 3.5 billion years old. These diamonds were forming in the Earth's mantle before oxygen arose in the atmosphere and before there was life as we know it on Earth.
These incredibly old ages will never be matched in any synthetic lab-grown diamond. Though lab-grown diamonds are cool for completely different reasons, their prehistoric ages is not one of them.
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To read more about the Ellendale diamond ages, go here.