Diamonds form mainly in two types of rocks. One is peridotite. What is the other?

• A. Eclogite
• B. Granite
• C. Basalt
• D. Gabbro

Answer: (A)

Diamonds form in environments of very high pressure deep in the mantle, so the rocks that host them are linked to those conditions. Besides ultramafic peridotite, a second rock type commonly associated with diamond formation is eclogite, a high-pressure metamorphic rock that develops from basalt in subduction zones. Eclogite forms under the same extreme pressures where diamond stability is favored, and its presence indicates the deep-melted, carbon-rich conditions needed for diamond growth.

Granite, basalt, and gabbro sit in crustal or shallower mantle settings and do not typify the high-pressure mantle environments most closely tied to primary diamond formation, which is why eclogite is the best match alongside peridotite.