Peridotite

Peridotite properties at a glance

Geology & background

Peridotite forms the bulk of Earth's upper mantle and reaches the surface through several processes: as xenoliths carried up rapidly by kimberlite and basaltic magmas, as large slabs of ancient oceanic mantle called ophiolites thrust onto continents during collision, or as exposed massifs where mountain-building has uplifted mantle rock directly, as seen in Norway's Lofoten region or Spain's Ronda massif. Other classic localities include the Josephine Peridotite of Oregon and California and the mantle sections of the Oman ophiolite, one of the best-preserved slices of oceanic mantle exposed anywhere on Earth. Because it forms at great depth, peridotite weathers rapidly at the surface, often altering to serpentinite.

Rockhounds should know that fresh, unaltered peridotite is relatively uncommon at the surface precisely because it weathers so readily, and much of what is found in the field has already been partially serpentinized, taking on a greasy, mottled green look. Peridotite is also important as the host rock for diamonds carried up in kimberlite pipes and for certain nickel and chromium ore deposits, adding economic interest beyond its value as a mantle sample.

How to identify Peridotite

Peridotite is typically dark green to greenish-black and coarse-grained, dominated by olivine crystals that give it a granular, sugary texture, with pyroxene present as darker, blockier grains. It has an overall hardness around 6 to 7 reflecting its olivine content, no rock-scale cleavage, and it commonly shows signs of alteration to serpentine along fractures, producing a mottled or greasy green appearance. It is distinguished from basalt by its much coarser grain size and lack of feldspar, and from gabbro by its dominant olivine content and darker, more uniformly green color; heavily serpentinized peridotite can be mistaken for serpentinite itself.

• Compare the specimen's color, luster, crystal habit, grain size, streak, and locality before confirming.
• Use multiple photos when possible, including a fresh surface and a close view of texture.

Where Peridotite is found

Reported from multiple localities worldwide; verify locality against specimen provenance when available.

How much is Peridotite worth?

Value depends on size, crystal quality, color, locality, rarity, damage, and provenance.

Common uses

• Field identification
• Construction or aggregate when suitable
• Landscape interpretation
• Education

Interesting facts

• Peridotite is the dominant rock type of Earth's upper mantle, meaning most of the planet's interior by volume is essentially this single rock.
• Diamonds form deep within the mantle in peridotite and are carried to the surface as fragments within kimberlite pipes, which is why peridotite xenoliths are studied for diamond exploration.
• Gem-quality olivine, known as peridot, is essentially crystallized fragments of mantle peridotite ejected during volcanic eruptions or found in peridotite outcrops.

Care & safety

Handle as a collector specimen until hardness, cleavage, and fracture are verified.

Avoid inhaling dust. Do not cut, grind, or polish unknown specimens without eye protection and a proper respirator.

Related rocks & minerals

Quartz Quartz Family Feldspar Feldspar Group Andesite Igneous Rocks Anorthosite Igneous Rocks Aplite Igneous Rocks Basalt Igneous Rocks