Lamproite

They also vary widely in age, from Proterozoic to Pleistocene, the youngest known example from Gaussberg in Antarctica being 56,000 ± 5,000 years old.

The presence of the following minerals precludes a rock from being classified as a lamproite: primary plagioclase, melilite, monticellite, kalsilite, nepheline, Na-rich alkali feldspar, sodalite, nosean, hauyne, melanite, schorlomite or kimzeyite.

This discovery led to the intense study and re-evaluation of other known lamproite occurrences worldwide; previously only kimberlite pipes were considered economically viable sources of diamonds.

Research at Argyle diamond have shown that most stones are of E-type; they originate from eclogite source rocks and were formed under high temperature ~1,400 °C (2,600 °F).

The diamonds occur as xenocrysts that have been carried to the surface or to shallow depths by the lamproite diapiric intrusions.

Modern terminology classes all as lamproites but modifies this term with the mineral abundances as per the standard IUGS rules.