Which high-pressure metamorphic rock is associated with deep Earth diamond formation contexts?

• A. Eclogite
• B. Schist
• C. Marble
• D. Sandstone

Answer: (A)

Diamonds form and remain stable at very high pressures deep in the mantle, so rocks that record those high-pressure conditions point to the environments where diamonds form. Eclogite is a high-pressure metamorphic rock that develops under those extreme depths, typically from basaltic material in subduction zones. Its formation at such pressures (and its characteristic garnet + omphacite assemblage) makes it the best indication of the deep mantle context where diamonds originate. The other rocks—schist, marble, and sandstone—form at lower pressures or are sedimentary/low-grade metamorphic, so they don’t embody the same deep, high-pressure formation environment associated with diamonds.